I use a nautilus machine at the gym and I think its called the torso machine, or something like that. Hard to explain but youre "twisting" each side of your body as you straddle the pad/seat. Your arms are wrapped around a bar on each side but youre using your obliques to turn and lift the weight. You do one side then switch to the other. It has definitely helped tone my sides.
Also, side crunches do help. It works best if you use a decline bench. Lying on your side w/ your knees bent, hold onto the top of the bench and slowly bring your knees up to your chest. Do one side then switch.
Hope this helps!