Recently, a client emailed to ask how he might improve his double-under abilities. We told him this:
When people learn to kip pull-ups, they often try to chain them together before they're ready. The first pull-up looks pretty good, but because their technique isn't quite perfect, they end up 10% horizontally displaced from where they started the first rep. After the second pull-up, they're 20% out of place. By the third, they're simply flailing around while holding the bar, no longer doing anything that even vaguely resembles a kipped pull-up.
The solution there is the same one that helps on double-unders, which we think of as the 'n+1' rule of fitness:
If you can't do one really good rep, the second one isn't likely to be better; if you can't nail two, three is going to be a mess; etc.
Start with the goal of nailing a single double-under 100% of the time. The easiest way to practice that is with two leading singles (to get the rope moving) and with one trailing single (so you don't get in the habit of stopping turning the rope after you hit the double). In other words: single-single-double-single.
Do this for a week or two, until, again, you hit the double 100% of the time. Then add a second double-under: single-single-double-double-single.
When you're at 100%, add a third. Keep adding them this way, one at a time. Once you can get to five or six, with perfect reliability, you have the movement down. Now you'll just be limited by endurance (and, relatedly, efficiency of your technique).
One other point: double-unders definitely yield best to daily practice, though you should make a point of doing some of your practice sessions *after* your WOD. A lot of people can do double-unders until the cows come home when cool and collected, but totally lose the ability once they're fatigued and their heart rate is up. Post-WOD practice sessions are a great way to make sure you can keep crushing it in battle conditions. ;)