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.