This exercise from Jen Widerstrom, a trainer on The Biggest Loser, is great for shaping your thighs and butt. Remember to put equal weight on both legs so you move straight up and down, not forward.
Here's how to do it: Start with your feet hip-width apart and hands on your hips. Step your right foot forward, coming into a wide stance with both feet pointing forward and your left heel lifted off the ground. Bend your knees, lowering until both legs form a 90-degree angle. Do 10 reps. On the last rep, remain in bottom of lunge and pulse 10 times. Repeat sequence on opposite side.