I agree with CK. You gotta hit the abs heavy. Weighted crunch, roman chair, heavy machine crunches, weighted leg raises, planks, do the trick and with less reps.
It kills me to get on a bb site with some dumbass personal trainer saying that "everyone has a six-pack, you just gotta lose the layer of fat to see it". Really? Then why don't anorexics have nice six packs, or survivor contestants after 39 days? lol. Sure, lose the mid-section fat but you gotta go heavy to make those muscles pop. 50 crunches off a ball ain't gonna do it.