I recently attended a seminar that Tom Platz gave and he gave me an ab routine for days off from heavy lifting. Basically I am now doing this ab routine every other day.

Crunches 3 sets of 50
V-ups 4 sets of 50
Hanging leg raises-3 sets of 50
Holding in Yoga- boat position 5 minutes, short rest 5 minutes short rest and last time holding as long as is possible--usually I start cramping up after 7 minutes
Incline sit-ups 1 set of 100
Leg raises--holding feet 2 inches off the floor---as long as I can, -usually at the end of my ab workout I cannot hold my feet off the floor for more than 5 minutes, take a rest and do another time until the 3rd time then hold as long as possible.

Finish with 2 sets of 30 reps bent knee sit ups light incline