the abs are link any other muscle in your body... i know loads of people who do 100+ reps to work their abs but it does nothing good. you wouldn't do 100 bicep curls?
i train my abs hard once a week and it consists of:
situps, 12 reps, 3 sets
crunches, 8 reps, 3 sets - using a 5lb weight
hip raise, 20 reps, 1 set - for the lower abs
hanging leg raises, as many as i can do, 1 set - for the lower abs
protein shake, done.