the best one, if you want to watch TV while you're doing it, is:
Lie on your back, calves resting on the ball. Pull your body up to a bridge (obviously bearing in mind usual safety and good sense thingies about overarching your back). Keep it there for a count of -ooh, five, ten, 20 - whatever your fitness dictates. Then slowly return to earth and repeat ad nauseam. The natural wibblywobblyness of the ball while you're trying to stay still and in the right position does wonderful things for your core muscles.
