RP33 is right use weights for your abs, also make sure you hit your lower, upper, side, part of your abs. Theres so many different ab exercises theres Bicycle crunches, which is 250 times more effective than a normal crunch, theres leg raises, hanging leg raises, ball crunches, dumbbell side bends, ETC.

There easy to find on the web the most effective ones, but just remember doing all these ab exercises is all well and good. But if you don't do enough cardio, or your diet isn't healthy. You won't be seeing no six-pack, even if you did 1000's of crunches.

You need to get your body-fat at atleast 10 percent or lower, to see your abs. So make sure you do lots of cardio 30 mins 4 times a week or so. Look up online a good steady diet for you. Remember the old saying abs are made in the kitchen not in the gym, and even though you have to do ab exercises to shape your abs. Just remember thats only part of it.