Check out this exercise which focuses on your glutes and hamstrings. This move appeared in the 704 issue of OptiMYz as part of the Build your booty workout with Ashley Pacht.
SWISS BALL LEG CURL
A Start by lying on your back with your palms facing down and the back of your calves resting against the ball.
B Lift your hips off the floor, so that your body forms a straight line from shoulders to ankles.
C Pull the ball in with your feet and then continuously push it back out.
Advanced: To work your butt and hamstrings harder, extend one leg straight up. Drive your hips towards the ceiling while moving the ball.