I do my abs every time I go to the gym which is about 4x week.
At the end of my workout i'll..
do 4 x 20 reps on this machine that has weights on it to add resistance
then 4 x 20 reps of hanging knee-lifts
iff i'm not too sore, 4 x 20 reps of crunches w/ medicine ball.
If you've got a partner, sitting up and tossing them the medicine ball, then having them toss it back is a really good workout i find. but rare for me cuz none come /w me to the gym.
Another good one to do if you've got a partner, is to lie down on your back,
have your spotter stand with your head between their leggs and grab their ankles. then just lift up your legs in a vertical position and your spotter will try to throw your leggs to the ground. you'll resist it of course so your feet dont hit the ground, and when you've reached the end of the motion, bring them back up to a vertical position to repeat another "rep"