This instructional video from Men's Health demonstrates one of the best ways of toning your abs. Start out with an easy weight and progress to a heavier ball for more effectiveness.
Lie on your back holding a medicine ball in front of your chest, elbows flared out to the sides. Bend your knees slightly and curl your torso up so your upper body is at a 45° angle. This is your starting position.
Bring your left knee towards your chest as you straighten your right leg and simultaneously twist your upper body to the left until your right elbow meets your left knee. Reverse the move, drawing your right knee to your left elbow. Try to alternate without resting until you reach your prescribed number of reps.
The bicycle move has been shown to be the best for working the abs and the obliques in studies by the American Council on Exercise. To advance, simple try using a heavier medicine ball and repeat the technique.