Here's a great one for toning the tush and backs of the thighs.
- Start by lying flat on your back with an exercise ball under your heels. Bridge your hips up, and hold that position through the entire exercise.
- Flex your feet and dig your heels into the ball. Place your arms straight out to your side for support — don't use them, though; they are just for support.
- Slowly roll the ball toward your body as you curl your heels. Keep your hips in the bridge position; don't thrust them toward the ceiling as you do the move.
- Then slowly return to the start position (straight legs, hips bridged) to complete one repetition.
- Do three sets of 12 to 15 reps.