Here is a great exercise for the glutes! Thigh abduction (movement away from the midline) is one of the best ways to work the butt. If you're an athlete, this exercise can actually enhance lateral movement speed and power. Instructions for the seated thigh abduction with O-loop are listed below:

Sit down on a workout ball and secure the resistance loop just below your knees. Position your feet approximately 12 inches apart. Keep your feet stationary, then slowly widen your knees as far as you can. Range of motion will be relatively short. Return to the starting position. Be sure to control the speed of resistance loop as your knees return to the center.