Originally posted by: eits
dude... the absolute killer of ab workouts = pulse-ups. they sound simple, but they KILL you. i dare you to do 20 controlled pulse-ups.
http://www.differ.freeuk.com/learning/health/bkreverse.gif <--- these are kinda like pulse-ups, except you make sure your legs are perpendicular with the floor at all times. this guy looks like he's rocking back and forth on his back, kinda. with pulse-ups, you pretend that there's a button about 6 inches above your toes and you need to touch it.
keep your hands at your sides right next to your butt, but not under.
Wow, I did those with a friend holding his hand about 6 inches about my toes, and a little closer to my head (like a button). I had to hit his hand with my feet in a controlled manner. We both loved that exercise, and it really works your abs well.
Another exercise I enjoyed was: Lying on my back, legs straight and arms at side. Lift legs up to 90 degree angle with ground/body and then have a friend push your feet back down to the ground. Don't let your feet hit the ground, but come close. And repeat. GREAT exercise imo.