I used to have an 8-pack, from just doing crunches...
I'm working at getting it back now. When you do crunches, the key is not to do a lot of them until your abs get numb. Instead, aim to do only 12-15 reps, but you have to do them in a way that you reach failure at that point. When you move your chest up, exhale and tighten your ab muscles, but don't loosen them on the way down. Also, when you're at the peak of moving upwards, do a couple of mini-reps...as in, slightly move back down and back up, inhaling and exhaling as you do. I usually do 3 mini-reps for each rep. And move down slower than you move up. I do 3 sets of 12-15 reps.
I guarantee you'll feel the burn if done right. I don't remember where I read this, but these are apparently called "killer ups" or something like that. Don't work abs everyday...like 3 times per week is better...because the muscles need rest like any other body part.