I've been pretty fortunate, nearly zero body fat, so there's always a 6-pack showing even if I haven't worked on it in a long time. When I do work on it, I found the best way is to lie down on your back and lift your feet 6 inches off the ground and hold it there as long as possible. Can be further expanded upon by repetitions of 1-feet up 2-feet apart 3-feet together 4-feet down. Nothing burns 'em up like the foot lift thing for me.