Use Your Glutes
During intercourse, your butt is functional and well as decorative. When you thrust your hips forward, you use your glutes. More powerful glutes equal better thrust. Squats, a weight-room classic for butt toning, also work your quadriceps and hamstrings. To perform a perfect squat, hold dumbbells or place a weighted bar across your shoulders. Stand with your feet about hip-width apart and parallel. Keep your belly in and your eyes facing forward. Slowly, sit your butt back and down, as if reaching for an imaginary seat behind you. Lower yourself down until your thighs are parallel to the floor (or as close as you can get them) but no lower. Your knees should not shoot out past your toes. Now, squeeze your glutes together and slowly straighten your legs until you are standing in the starting position. Start with two sets of 12 repetitions and work your way up to three.
Strengthen Yourself on Top
Being on top in the "missionary" position can be physically taxing. In a nonsexual context, it's actually an isometric exercise called "plank position." To practice it, lie on your stomach with your bent elbows directly under your shoulders and your toes curled underneath you on the floor. Draw your belly in toward your spine and push yourself up, keeping your hips in line with your shoulders. Your legs should be straight. Your body should form a straight plank with no break at the hips or shoulders. Hold this position, continuing to breathe normally, for at least 20 seconds and as long as possible.
This exercise works your lower back, your abs and all the "core musculature." If you concentrate, you can even feel your PC muscles clenching along with the deepest layer of your abdominal cavity.