Running downhill is key to building strength in the legs, and the uphills challenge your aerobic and muscular endurance. Running trails also improves your lateral agility and balance. Try to incorporate side steps and zigzagging. Hot-footing it over rocks and roots strengthens the support muscles in the lower leg, improves balance, and gives you quick feet. "We take our skiers out to dry creek beds, where the rocks are unstable," says U.S. Skiing's Andy Walshe, "because that adds a strong balance component and helps ankle and lower-body proprioception." Just like in skiing, you want to separate your upper and lower body. Notice that ski patroller Bradley Richmond's upper body and head are upright and facing downhill while her legs angle around the turn.