The posterior chain (upper and lower back, triceps, hamstrings, glutes) is key not only to sculpting a sexier flip side, but also to toning your front-facing parts. By focusing on the posterior muscles, you can lessen your chance of injury and improve your posture by drawing the shoulders back, turning the arm muscles outward, and lengthening the abdominals.
Stand with feet shoulder-width apart and hinge forward from hips (A). Using your body weight as resistance—add weights when this is too easy—raise arms to sides until parallel to the floor (B). Squeeze shoulder blades together, then lower arms back down. Repeat.
Sit with heels of hands on the edge of a chair seat. Slide butt forward and place heels hip-width apart on the floor, legs straight. Hinge from the elbows as you slowly lower body 6 to 10 inches (A). Press arms back up, contracting triceps (B). Repeat.
Lie face-up, put heels up on a stability ball, and lift hips up off the floor, forming a straight line from shoulders to heels (A). Keeping hips high, bend at the knees and roll the ball in toward your butt and then back away from you (B). Repeat.
Lie face-down with arms and legs straight and extended (A). Keeping your neck relaxed, simultaneously lift arms and legs toward the ceiling (as if you were flying) so that your body looks like the letter X from above (B). Tap the ground with your hands and feet between every rep.