My fav side oblique exercise:
-Start by lying on your side, supported on one forearm, and the side of one foot.
-Make sure your body is in a straight line.
-Hold for 60 seconds, switch sides, and repeat. I usually do 2 sets with my ab routine
-Alternately, I've seen people lower and raise themselves sideways in this position. Never tried it myself, as I like the isometric version better.