As you stand up, slide your left leg back in, bend your knee, and raise your leg directly in front of you to hip level. Engage your stomach muscles to keep your balance. Do 15 reps on each side, up to three sets. Too easy? Close your eyes, or hold a nine- to 15-pound body bar in front of you. Too hard? Get off the Bosu and slide your foot on solid ground. The Payoff: "Often when you're skiing, you either get too far over on the inside ski, or you hit a soft spot and lose balance, says Hattrup. "These help keep you stable.