My brother's friend found my 6ft On Guard cable lock and promptly reset the combination to something he was too lazy to remember. so I cam up with this method to find the lost combination and save the world (my world)
This is the semi-hard way. it's not the exhaustion method (trying every possibility) but it takes a little brain power and reasoning.
I know there's a lot of stuff on line about how to unlock a cable lock but these methods only work for the drug-store variety of locks that have low tolerances and a lot of wiggle room. (i'll explain why later) This one works on the nicer kind. the ones that have half inch braided cables and much tighter tolerances.

Footnotes:
Don't try to use this to steal a bike. It won't work. you can't put enough tension in the system to feel the shape of the disc using your muscles while rotating the tumblers. If that's your reason for being here, kindly piss off.

Edit (2014): I have since retired this lock. After moving to the Roger's park 'hood of Chicago, it has become rapidly apparent that cable locks are not only easy to crack but easy to cut. Kryptonite's higher-end locks still prove to be beyond my ability to crack. their tolerances are tight making the lock seize under tension. the only way to open one seems to be through exhaustion. anyone good with arduino want to make a robot?

I now use a keyed on-guard U-lock.

Step 1: Understand What's Inside

inside there are four rotating plates with a notch the shackle to slide through. When you scramble the lock, the plates slide to block the shackle from coming out.

Step 2: Tension the Lock

I wrapped the lock around a chair (pictured) sliding the cables apart increases tension in the system. you need tension to press the teeth of the shackle against the lock discs. this lets you feel the shape of the discs better and decide when one of the teeth isn't pressed against steel disc and is just hanging out in open air.

Step 3: Rotate the Tumblers

Start with the tumblers that are hardest to rotate first. these have the most tension. tension makes the disc's shapes easier to determine.

Now, there's a couple of things to look for. companies tend to put notches in the discs to give would-be thieves a hard time so you need to look for a union of events.
   -The tumblers will be really hard to turn under tension. When it's all of a sudden easy to move, that's an indicator you have one of the numbers
   -A previously loose tumbler just got hard to move. this means that tension was just taken off of one of the discs and now the shackle's teeth are pressing harder on another disc.
   -Any consistent response from the lock that is larger when one number is reached than any other. this can be a sensation or a sound. in more expensive locks (like mine) you'll feel (and excuse the hokey onomotopeia) griiiiiiiind...click...griiiiiiind.....jerk....CLICK. the last click (if you didn't guess) was likely the disc reaching it's correct place and a tooth popping into the gap.
   -It's harder to leave a number than it was to get to it. a tooth is in the way of the disc sliding locked again. stop touching it. you've probably got it.

None of these are sure fire on their own. There's a lot of money in making sure you can't figure our how to open a lock without knowing the combo. sometimes there are two numbers on one tumbler that seem likely. write them down and try them with the likely numbers from the others.

Step 4: Have Nerves of Steel

When you get it, you'll be concentrating hard and i can guarantee the lock will fly apart. Its going to startle you. Don't make this remove the idea of setting the combo to something you'll remember from your head.