Well, actually, I can only put one foot behind my neck at a time. Basically, I stand on one leg, take the other in hand, and move it behind my head. And then hop about on one foot. Works with either one. Very fun, especially as a way of terrorizing the populace.:)

Another fun thing to try is to hold out your hands in front of you, join them, and then swing them back over your head. If you go back far enough, people start calling you double-jointed and encouraging you to do it as a party trick.^-

I should post a photo, but I am too lazy to take one now. Easier with a friend anyways.