This is one of my all time favourites for working your glutes. You can make it harder and see quicker, more defined results by using ankle weights or an extension cable.
Start by getting on your hands and knees. Make sure your hands are parallel underneath your shoulders and your abs are held in tight. Start by lifting your right leg back and up making sure that your foot is flat facing the ceiling. Lift up and hold for a few seconds before dropping your leg slowly down. Do 3 sets of 20 repetions on each leg.