DISCLAIMER: Generally, it is the inexperienced folk who need the most advice on this kind of thing. However, some 'techniques' described are dangerous, decrease your safety margin, etc, so might qualify as the sort of thing best applied by experienced climbers. If in doubt, for f*cks sake- stay on the ground or walk off!
1) Use the EDK to tie two rappel ropes together. (Euro Death Knot. Death* Knot. Overhand. Whatever you call it works fine). It rides over rock features infinitely better than the old school knots, and conveniently is a hell of a lot faster to tie and untie.
*Why is it called the 'death knot'? It's probably more accurately called an overhand, or a double overhand, or who knows what, but 'EDK' was one of its more memorable calling cards when it hit the stateside climbing scene. And compared to all the old-school rap knots, it looks straight-up scary. Anyways, don't take it lightly because if you f*ck it up you could die. Unlike the rewoven figure eight and fisherman's knots, there is not much margin for error with the EDK. The EDK, tied incorrectly, DOES fail at loads that could be achieved while rappelling.
To tie the EDK, put the two ends of the ropes together. Tie them in an overhand (pretzel) knot with a good chunk of tail. I use at least a foot and a half. Other folks use other lengths. Just don't let it be short, as the knot fails by rolling over itself again and again towards the end of the rope. Nice to have a little margin there. Do not tie a figure eight knot, as it can roll at lower loads. Then make sure the knot is 'dressed'. That is to say, all the strands lay flat and parallel. There should be no kinks or twists in the knot. Then set the knot. This does not mean just grab two strands in each hand and yank. This means take each individual strand (there's four) and pull hard on it, one at a time. If you are unsure of what all this means, tie one and see if it makes sense. If you still arent sure, look up some pictures online or find a kind and enlightened climbing friend to show you.
2) Know which rope to pull. You can't pull an EDK in standard diameter ropes through a rap ring. A lot of climbers get overly worked up about this- it is usually immediately and readily obvious to even the dimmest climber when you are pulling the wrong rap line. However, in the rare case that you are rapping a really high friction face, it is nice to know that the reason the rope isnt coming down is because of the friction, not because you're tugging on a jammed knot.
This would probably be an appropriate time to mention that you need to be sure there are no knots in the end of the ropes when you are pulling the rap lines. Funny how often this seems to happen, and overhands, figure eights, slip knots, and other monstrosities have an odd habit of tying themselves in the ends of ropes when no-one is paying attention. Keep an eye on the end of the rope until it is out of sight/reach.
3) When you begin the rappel, make sure there are no twists in the rope above your device. When you end the rappel, do not let the ropes twist together before pulling. The additional friction can make it that infinitely (or impossibly) harder to pull a rope over featured, high friction, or slabby terrain. Yet worse, if you have a twist in the top of the rap, and the rope sits in a groove, the pull strand may become jammed when the knot tries to pull through but is pinned by the slack strand.
4) On windy days, wait to toss your rap line in lulls in the wind, or better yet- rappel with it. To do this, either pack-flake it into a backpack and just let it out as you go, hang the backpack off your harness beneath you, or flake it over a sling and hang that off your harness (the latter is faster, easier, but more likely to make a mess). Either way, you pay it out as you go. It takes longer, but it beats the hell out of throwing your rope and watching it wrap around the tower, only to get stuck on a chickenhead that is impossible to rappel to without a life (and/or ego) threatening pendulum/tension traverse/unprotected climbing/etc . Add wind noise, and it gets hairy. Believe me.
5) Consider rapping without knots in the end of your ropes. If you are rappelling very catchy featured rock, occasionally the knots will hang up somewhere on the toss that is tricky or impossible to get to, or to get unstuck. If you understandably want the safety margin that knotted rap lines provides, consider a rappel backup instead, which I think is a more logical safety measure anyways (If you drop the brake lines, would you rather stop immediately, or at the bottom of your rope?). Just pay attention to where you're going when you're near the end of the rope.
6) Watch the rope line. As you rappel, examine the rock you pass over. Are there cracks in it that will eat your rope? Chickenheads? Upwards facing flakes or slots? Pieces of old cut rope sticking out of a crack? Bushes? Cacti? All these are rope eaters or destroyers, and if possible, it would be nice so that when you pull your rope neither strand goes near them. Sometimes they are impossible to avoid, but generally you can do something with the rope as you rappel to keep it away from the rope-eater, using rock features to redirect the rope above you like putting your rope in a friendly groove or hooking it around a chickenhead to keep it away from the bottomless creaky flake which has seven frayed rope pieces sticking out of it.
7) Seal off rope eating cracks, if possible. I've had some success defeating otherwise ravenous upwards facing rope slots and flakes by lodging a few small rocks in them on the way down, or more normally, examing the rope-eater on the first rappel, and telling the second rapper to bring down some appropriately sized rocks/sticks/etc with her.
8) When there is a shelf, incut rock feature, constricting crack, ledge, or other obstacle that is likely to hang up the knot near the top of a double rope rappel, you would do well to clear the knot on the way down. This requires some fiddling, and wont work if you need to rap the full 50m, 60m, whatever it is that you are rapping. To clear the knot, rappel past the obstacle and stop at a convenient place where you can unweight the rappel. A ledge is best, but clipping into a bolt or solid piece of gear will work if you keep your head together and keep your hands on the brakes. When you've unweighted the rap lines, start feeding the ropes in opposite directions through your device so as to move the knot down. If the knot does indeed look like it will catch, either flick the rope around a bit and get it below the offending feature without catching, or get back up there and do something else progressive. It is much easier to deal with a knot-grabbing feature when you are only six feet away, not at the end of your 200' rap and you can barely even see the knot, let along flick it out of the crack. When the knot is below the feature, weight the lines again and rap away. Again- this is generally only done for features near the top of rappels (like clearing the edge of a belay ledge, etc) because they are more predictable (you know pretty much where your rope will pull over the rock near the top) and because it would take really long ropes to be able to clear the knot a long distance and still have enough rope on the threaded line to reach the next station.
9) Test pull. After your first climber has rappelled, if there is any question about whether the rope will pull (there is a feature near the anchor which looks grabby, or there is a lot of friction), the first climber should do a test pull where they pull a few yards of rope through the anchor to see if everything is working the way they intended. This can also clear up whether knots will clear ledge edges and whatnot. If everything is hunky dory, just even the ropes out again and the second climber can come down without worrying about whether it will pull.
10) The time the rope is most likely to get stuck, particularly if you take care to watch your rope line while rappelling, is on the pull, where the last chunk of rope whips through the anchor and falls. In order to minimize it getting stuck on rock features, wrapping around bushes, etc, pull slowly and gently when the rope is almost about to pull, and then when you can feel it just about to fall on its own weight, give it an enormous yank and watch it fly out and away from the cliff on the way down. At least that's how it works when the timing is right and the planets are correctly aligned. (Actually, its a good thing to do, whether or not your planets are aligned). one note: this violent rope whipping motion makes it anecdotally more likely to knot a twisted rope around itself at the rings. if your rope is kinky, or the pull is so low-angle that there is no way it could fall to you with even a hard yank, then just pull it through gently and cross your fingers.
11) Stand away from the rock when pulling. Obviously not an option in the middle of a multipitch rap, but if you're on the ground, you would do well to stand at least a few yards away from the face when pulling the rap lines. It will help you see what is going on, will keep at least one rope out of the cracks, and gives you a better shot at getting the last-second yank to work out well.
12) In rope-eating territory, in the middle of multi-pitch raps, consider flaking the rope as you pull it down from each rap, instead of threading it at the anchor, or letting it hang below you. Although it is thankfully rare, ropes do get stuck beneath climbers, and I have the dubious honor of having had a rope stuck both above me and below me, which leads you with fewer options for retrieval of the top line. If you flaked the rope while pulling it, and the top line gets stuck, you have the flaked bottom line as your new lead line, ready to go. Some climbers get pretty psyched about saving time on multiple raps by threading the bottom line through the anchor as you pull it, but I think this is overrated. With the EDK, it takes seconds to untie it, and only a few more to retie it, so with awkward anchors its sometimes easier to just flake it in a lapcoil, untie, rethread, and retie the ropes while pulling, instead of muscling the entire rope through the frictiony rings while pulling. When the top rope falls clear- just toss the lap coil off the cliff and you're all set.
13) Another option for rope-eating territory is to switch over to single rope raps instead of doubles. Obviously not an option for most places, but some times there is the option to do an intermediate rappel for the one-rope teams which two-rope teams happily bypass. If its a catchy pitch, rapping on one rope means no knot to catch, and only one rope in jeopardy at a time.
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Un-Stucking Stuck Ropes
This is the trick bit. Even the superhumans among us can't get it down sometimes. The first line of attack is to try to get it down without climbing anything. Sometimes this doesn't work either and you need to climb. A rough outline of strategies follows, although not all are necessarily used everytime, and many rely on you having both ends of the rope still with you.
0) Pulling with all your might is always the *last* thing to try. It is an act of panic or desperation. In some cases this will irretrievably fix your rope (bring a knife if it gets cold at night..). However, pulling with all your might is pretty intuitive. In fact, it works a lot of times. Just make sure you try it last, or in some cases, not at all.
1) So if you don't pull, what? First try to figure out what is going on. Did you do everything in the prevention list right? Try to remember if something didn't run straight or there might be twists up there or the like. Did you watch the tails? Sure there wasn't a knot in there when it left your sight? Can you see a knot at the anchors? Knots get left in ropes. They get tied when the rope kinks and twists on the ground or in the air. They get tied when the rope whips around. They tie themselves around anchors sometimes. Is the knot around the rings, or jammed in the rings, or does the rope just seem to 'end' at the rings? Sometimes with ropes with a stiff bit of tape at the end of the rope, the last inch of rope jams seemingly irretrievably, pinned between the rock and the weighted ring. Does the rope disappear behind a flake? How far can you pull? Are you sure you're pulling the right end? Try to figure out as much as you can. And then try to fix the problem. Techniques include-
2) Rope Flicking. Your basic unsticking tool. Works magic in most cases, particularly when you can see what the rope is caught on and flick in the appropriate direction, or off of or out of a feature. Sometimes rope flicking is a bit of an art, requiring deft coordination, so hand the rope to the intelligent person in the party. Seriously. Sometimes the double-flick is the way to go. Example- for long raps, or stuck ropes where the problem is over a ledge, sometimes a big whip won't move the rope around much at the top. If you give the rope a big whip, wait for the wave to move up to near the top of the rope, and then give it another huge whip, you'll often get a super-whip at the top of the rope. Try this (or have your girlfriend do it, she's smarter than you) and you'll see what I mean.
Generally, unless you know exactly what is going on, a dozen hard rope whips is a good way to start the stuck rope dance, and it only makes things worse if you are on super-featured-catchy-positive-chickenhead-terrain and you're not taking your medications.
3) Try untwisting. Can you get far away from the cliff to be sure your ropes are not twisted? Are they twisted above a ledge? If you're on the ground, you can often grab an end and walk away from the cliff to get a better angle. Try pulling from out there, maybe. Or maybe you and your partner take an end each and start doing mayday circles around each other to get the twists out. Sometimes you can't see your rope, but think it might be twisted. If you're desperate, you can put a couple twists in the ropes in one direction, flick it around a bunch of times to work the twists up the rope, and hope it made it better. If nothing improves, twist the appropriate number of turns in the appropriate direction to undo the extra twisting, shake it out, and try the other direction. This rarely does anything great, but you never know. Screw it up and you'll guarantee yourself a friction-stuck rope in slabby or featured terrain.
4) See-sawing. OK, I made that term up. But say you've pulled down a few yards of the rap and then it jams, or starts to get tough to pull. Want an 'undo' button? Your 'control-z' at the crags is to pull the rope back to where you started. Then flick it around a bit, and try again, and again. Obviously, this won't work if you no longer have both ends in your hands. Which begs the comment- If you're having trouble pulling a rap, you better feel really good that its going to work out before you let go of the non-pull strand.
5) Are you sure you're pulling the right rope? When desperate, try the other one. You'll be pulling rope and you'll get a sudden jam feel, which is different from the gradual stretchier jamming of a rope getting embedded in a crack.
6) Pulling. Not to be confused with Pulling With All Your Might (PWAYM). We all do it. Generally, be conservative. It is a good tool to check whether the rope is going to come despite the twists, or to get it off of a chickenhead, or whatever, but remember that anytime you pull the rope without being able to see what is going on you are taking a risk. Generally, I'll use a handful of firm fishing tugs to check what's going on up there, but remember- you're just checking up on things. If the rope gettings harder and harder to pull with each tug, then stop tugging already! You are jamming the rope and are probably making things worse.
7) Ah, it's what you've been waiting for- the PWAYM. You can try it as a last ditch attempt, except in one of the following two situations: Do Not PWAYM a stuck rope that could be wrapped around a loose block above you. It may free the block, which will then fall on you, simultaneously killing your partner, chopping your rope three times, the anchor, and scratching your sunglasses. Your partner may survive, but only if he escapes anything but a glancing blow by running through a field of prickly pear cactus (Jeff, I'm very happy you're still alive.). Second, Do Not PWAYM a stuck rope that appears to be in a rope-size flake. Particularly not one that already has pieces of tat rope sticking out of it. Of course, if you are on the ground and do not want your rope, go ahead. I guess. It's not Leave-No-Trace, but hey- think of it as a contribution to the local dirtbags.
One note on flakes, while we're at it- when the rope is getting jammed in/behind a flake/crack, pulling down may weld it. Unless you're sure it's going to work out, consider assuming the rope is not going to pull, and figuring out some way to get up to it so you can better analyze how to extricate it. Generally sideways or upwards pulls are much less risky to the long term health of your cord.
If the rope is pulling, but there is so much friction that it takes all your remaining bicep, well, good luck. Borrow your partner's gloves, pull together, wrap it around your hands, take breaks and stay hydrated, and keep your fingers crossed because you are always taking a chance of welding the thing irretrievably.
It Didn't Work. I'm Going Up There.
Your rope is jammed, or you don't want to take the chance welding it with further pulling. OK. That sucks. But you're still alive right now. Before you start batmanning the fixed line, have a good think about things. Weigh the risks of not having your rope vs retrieving the rope and all the myriad Bad Things that could happen. This will be your judgement call in the end. Just be aware that plenty of people get broken, or completely written off, because of rope retrieval tomfoolery. That said, I'll mention a few retrieval techniques, but there is no one answer. Like building anchors and moving fast, you have to choose your own path. A few things you might consider: 1) Do you have both end of the rope? Yes? You're golden! Rig up the prussik slings you always carry on the back of your harness around both ropes with your favorite friction knot and jug the line. There are books and books on this, and I don't want to bore you, but make sure you have at least two knots between you and the deck (two prussiks, or better yet, a prussik or two and a figure-8 backup clipped to your harness). Get to the top, fix the problem, and rap. If you (for some inexplicable reason) carry Tiblocs instead of prussiks (they're nowhere near as versatile) then you'll need to tie off one end of the rope and jug the other. If on the ground, tie the fixed strand to your partner as a counterweight.
2) If you no longer have both ends of the rope, you have options. And most of them suck. The only remaining Non-Suck option is re-leading the pitch with another rope. This assumes you have a half-rope or another single or something that should hold a real fall. This is one of the distinct disadvantages of the dental-floss rap lines. Leading a pitch on 200' of static 6mm cord really doesn't do it for me.
The Mid-Suck option is to use the fixed line as protection and take it from there. First, PWAYM the hell out of the thing, say your prayers, and hook your prussiks up to the line. Then go up.
Note: Do not kill yourself attempting to retrieve the rope. it is embarassing and painful. just because a rope feels stuck, doesn't mean it's necessarily good enough to jug. Can you see what is stuck? Is there a knot jammed? Is it a real knot, or just a few kinks? Are the rings big enough to pull the knot through? Think, man, Think!!
You may choose to climb it, sliding your two prussiks up with you to provide a solo TR setup. You may choose to jug the line outright. You can protect yourself from dying should the unknown 'anchor' above blow out in a few ways. You could use another rope to lead the pitch with, putting in pro as per a normal pitch. This is very useful if the rap descent is down some 5.12 testpiece. You can't climb it, but you can jug the line, with a leading setup below you (requires another rope).
Finally, the High-Suck option. If you only have one end of the rope and you have no other rope to belay with, you can give yourself a little margin by jugging/climbing and putting in pro, then clove hitching the rope beneath you to the pro. (Because there is no way to feed you more rope, remember that you won't be able to put much pro in, so make your pieces good. You could put prussik slings on the rope to pieces, but they will likely be melted if you go for a big ride). With this setup, if the anchor blows, you might go a long long way, but hopefully you won't fall off the mountain.
Obviously, in this case, you need to have a real tie-in knot to the rope at some point (as previously mentioned, a big fall will destroy a prussik sling). A clove hitch on a couple lockers on your harness works, and you could feed it as you go up. A gri-gri type device would be easier (though slightly more dangerous), but you didn't bring one.
Anyways, as with everything in climbing, its all about judgement. Judgement in how to avoid getting your rope stuck. Judgement in how to retrieve it without welding it. And judgement about how to get back to the anchor without dying. A little practise and a little experience helps. A basic understanding of self rescue techniques is invaluable. Sometimes sh*t happens. Stack the odds in your favor. And remember: there's always bouldering.