I believe this is the way to get records. Has it always just been a matter of luck for everyone, or has anyone set out to deliberately do this? If there is some kind of technique to this, please share. ;-)
One way you can do this is hitting a creeper with an iron sword twice, and then run around until you find a skeleton. From there, run circles around the two until the skeleton (hopefully) hits the creeper.
Another way takes a bit more preparation but is safer. Make something like this . . .
. . . and lure a creeper into the middle pit, a skeleton into one of the sides, and then go into the other side. The skeleton will shoot the creeper.
(Now that I think about it, you could probably suffice with the pits being one square wide instead of two.)
I have always gotten them by accident. However, it shouldn't be too hard to make it happen if you know what you're doing.
If you have a flat area, it is fairly easy to kite creepers. You can run as fast as them. The other important thing to note is that the skeleton should only need to land the killing shot - it doesn't have to do all the damage.
If I was going for records, I would get to a high place, at night, with relatively flat areas around it (in my case, the tower at my main base). Wait a little while and look for where skeletons and creepers spawn. Go get a creeper's attention first. Kite him around, and get a few shots in with sword or bow/arrow. Then, head toward the skeleton you spotted. You need to strafe or circle the skeleton to avoid his shots, and the nearly-dead creeper will be following and get hit.
I believe the creeper will switch targets to the skeleton as soon as he gets hit, so he needs to be one arrow away from death, or he'll just turn and blow up the skeleton.
Obviously, there are several risky elements to this, so you may want to practice kiting creepers and strafing skeletons (seperately!). I'd also suggest emptying your pockets of anything important. You may want armor for survival, or you may want to ditch it, rather than use it up on record-farming.
As I mentioned in this question, it is definitely possible to get records by shooting the creeper with your own arrows, so that is another option.
Also, it is possible to find records in dungeon chests.
(there are currently only two records total)
Here are a couple ideas that might help:
Sadly thats the best advice I have. I do know you can get records from chests, and I also know that this song is awesome, but that's off topic.
My approach was a disc-farming attachment to the sky mob farm.
First, there's "selection": A water channel pushing the mobs towards the dead drop chute was augmented - glass walls and two pistons triggered through the same switch, one of them extending while the other retracts.
X - piston; | - piston rod (extended); # - solid block or glass; O - hole, @ - water source; ~ - running water, \ - switch
As seen from above, inactive state
First, I pick out a skeleton from the constant stream of the mobs and drop it into the hole.
Below the hole there's the distribution corridor ( ~ - the water source block - happens right before the O above):
By activating either of the pistons I make the water flow carry captured mob to left or right drop. I send the skeleton to the left set of holes, then close the water flow.
Next I pick out a few creepers and float them to right side hole.
Then I go level below, open the door and stand at the end of the corridor. Once the creepers are dead, I close the door, open the small fence gate and collect the records - then go to bring more creepers.
Then below there's the actual collection corridor.
= - fence gate; ] - door, % - fence; S - skeleton, C - creepers, P - player