for abs you need to eat well because most guys store fat on their stomachsOriginally Posted by Howard Stern
running will also reduce fat on the stomach area which will define the muscles beneath,
but since you said its already flat do a mixture of crunches (lateral and straight, inverted if you have a board) and also varying types of pilates type things are good
like lift your legs and your back. be resting only on your lower back, bend your legs and a 90 degree and bring your left elbow to your right knee isoloting your abds as you do it
then right elbow to left knee etc repeat