I've seen and heard about all sorts of crazy abs stuff, and almost nothing works for me.
For 2 (min) and 3 (absolute max) times a week, take an ez-curl bar. Put tens on each end. Get in a situp position with the bar held behind your neck and your feet secured under something. Now this part is important... elongate your body -- streatch out so your head is as far away from your feet as possible (don't ball-up like a crunch).
Do 8-10 reps, then add 5's or 10's to each side for each set. I'll typically do:
3 tens on each side for 10 reps
3 tens and 1 five on each side for 8reps
3 tens and 2 fives on each side for 6 reps
With your body elongated, you get maximum muscle work and it hits upper *and* lower abs. Just make sure you go all the way up and down, and when you are back on the floor, pre-streatch your abs before doing the next rep.
Works like a charm!