Target: Upper Abs
Lie on the floor with with your legs bent, feet flat on the floor. Gently cup your hands behind your head, but don't pull on your neck whatsoever. Using your abdominals, crunch your upper body toward your knees, bringing your shoulder blades off the floor. Squeeze your abs for a count, then lower your shoulders back to the floor and repeat.
Target: Lower Abs
Lie on your back with your hands underneath your hips. Raise your legs off the floor, keeping a slight bend in your knees. Powerfully bring your knees toward your torso, allowing your hips to raise off the floor. Slowly reverse the motion, allowing your heels to almost touch the floor before beginning the next repetition.