I get them once in a while, or I used to. Stretching the muscle out helps, which is why GfS's and MtheGM's ideas work. The problem for me is that I can't do that indefinitely, and when I relax, the cramp is back... then the other leg starts up.

What helps head them off for me is to move the leg around, in all directions, even the ones that feel like they're going to hurt. If everything fails, I just get up and stand for a while. I think gravity helps get the blood through those spasming blood vessels.