Try out the "side jackknife" exercise from Men's Health. The jackknife exercise really squeezes your obliques as you lift your legs. Great for your abs!
Lie on your left side with your legs nearly straight and slightly raised off the floor. Also lift your torso off the floor and keep your left forearm on the floor for balance. Hold your other hand behind your right ear, with your elbow pointing towards your feet.
Lift your legs towards your torso while keeping your torso stationary. Pause to feel the contraction on the right side of your waist, then slowly lower your legs and repeat. Finish the set then switch sides.
As you advance with this exercise aim to slow both the movement and the pause at the top. Really try to squeeze the obliques hard before returning to the start position. If this becomes too easy, ask your training partner to apply pressure at the top of your foot to help add some resistance as you lift your legs.