Train for Flat Abs: Bicycle
You want four to eight key moves in each workout. Be sure to target the upper and lower abs, the oblique muscles along your sides, and the back. The "bicycle" is one of the best. Lie on your back and "pedal" in the air. Raise one shoulder as if trying to touch the opposite knee. Repeat on the opposite side for two sets of 12 reps. Keep your elbows back and your lower back on the floor.
Train for Flat Abs: Classic Crunch
The crunch is a classic because it works. Lie on your back, knees bent and feet flat on the floor. Put your hands behind your head, elbows facing out. With your low back on the floor, use your ab muscles to lift your upper body as far as you can. Hold the pose, then slowly return to the floor. Do three sets of 10-12 reps. Rest for 30 seconds in between all ab exercises.
Train for Flat Abs: Reverse Crunch
The reverse crunch targets the lower abs, which are tough to tone. Keep your arms at your sides, palms down. Use the abs to lift the legs, bringing the knees directly over the hips. Contract the abs further and raise your hips and lower back off the mat, knees toward your face. Hold briefly before lowering back to the mat. Don't let your feet touch the floor. Do three sets of 10-12 reps.