A Bosu Balance Trainer looks like an exercise ball sliced in half. They're gaining popularity at gyms across the country because they're so versatile for cardiovascular and strength workouts. Andy Walshe, sport science director for the U.S. Ski Team, uses them with his athletes because they "benefit fine muscle control," he says. Squats, without weights, will definitely do that. Stand on the Bosu, dome-side up, with feet shoulder-width apart. Drop down as if you were sitting in a chair. Keep your head and chest up and your heels on the ball. Drop until you reach a near-sitting position, then slowly rise to the starting position. Repeat 10 to 15 times. To make the drill tougher, place the Bosu flat-side up. Over time, when you become comfortable doing two-legged squats, transition to single-legged ones.