Lately, I've been doing light ab work once a week, after chest and triceps. It's mainly to help them stay strong, since I feel that using heavy weights on the abs, like any other muscle, will make them grow larger, which isn't really what I'm looking for. Usually anywhere from 3 to 6 sets in total, with high reps (20 to 30) per set. Incline crunches (body weight only) are the mainstay, but I may also do cable crunches, leg raises (body weight only, for the lower abs) or even the ab machine.