For pronounced abs, all it takes is a very strict diet. However, you also want to make them bigger to help them show. This would mean to treat them like any other muscle. I agree with BradG that you need low reps with heavy weight for growth. I disagree with the latter BTW (Apoc)
Anyway, what works best for me is to use the exercise ball at your gym. Use the ropes on the low cable pulley and add some weight. Try and hit 4 sets of 10-15 reps. Just make sure that you're leaning all the way back on the ball to seriously stretch your abs. (I do 15 degrees back).
Another good one is russian twists. Pm mw if you want to know how to do them. We always did these in the Corps so it obviously works ;) IMO