yah doing infinite reps for abs does nothing for you. abs are mostly fast twitch muscles, meaning you have to hit them hard, and fast for best results. 3 sets of 15 with no more than 5 seconds of rest in between works best, and adding some weight never hurts(you don't bench a plastic broom-handle, so why work abs out with no weight?). and try to get in 2 or 3 exercises.