The Westside routine I'm currently on has me working abs twice a week. I pick one exercise like standing ab pulldowns, straight leg crunches/situps, straight leg spread eagle crunches/sit ups, or straight leg raises, and do 3 sets of 10 (or something like that).
Also, exercises like GM's (and all variations), deadlifts (all variations), and squats (all variations), will work your abs; especially if you don't use a belt.