Double Unders my nemesis.  I know it takes practice, although, Buddy Lee makes it look so easy. I think one of the deteriants to practice is the whip marks on your arms and legs from missing. Anyway check out these video, maybe one of them will click:

http://www.youtube.com/watch?v=1yHjnfABvhw

http://www.youtube.com/watch?v=Ix8TbKyvJcw