To ski strong, you need to be strong. Strong like bull. The Rock Pushup is a great way to improve core strength without spending an infomercial penny. Place your hands on a large rock, shoulder-width apart. Stretch your legs out behind you, feet comfortably apart. (Think about forming a triangle with your body.) Walk your feet back, away from the rock and hold a pushup position, keeping your shoulders, hips, and feet in a straight line. Try to get your feet as far away from your hands as possible while keeping your body in good alignment.