Updated December 16, 2014.
Another ab exercise to add to your routine is the Hip Lift. It strengthens the rectus abdominis (the muscle between the ribs and hips), and the obliques (muscles that run down the sides of the torso). Try to work up to two sets of 10 to 12 repetitions, with a short break between. In addition to other ab exercises, you can do this several times a week.
Hip Lift
Begin by lying on your back with your arms by your sides, palms facing up.

Raise your legs so they are straight up toward the ceiling and perpendicular to your torso.

Pull your navel toward your spine and lift your hips a few inches off the floor, keeping your legs pointed straight up. Then slowly lower your hips back to the floor.