Compound excercise will make you much stronger (and bigger, if that's what you want) than isolation exercise. For the most part, forget isolation excercises like leg extensions, concentration curls, and most excercises done by machine, and do big-boy compound excercises like squats, deadlifts, bench, pull ups, dips, and military presses. The latter will make you much stronger overall and for skiing in particular. They'll also give you a much more efficient work out. Of course, working out your core muscles (the muscles around your torso) is also important for skiing. Last, use perfect form and don't go too heavy too soon, especially for squats and deadlifts.