I'm no expert but I think that in additon to strength training (squats etc.), and lactic acid tolerance? (wall sits), plyometrics really help. They give your muscles power, which is the ability to fire quickly while under load. Exactly what you need on the slopes. In the gym, your body is fairly static, and you isolate a muscle group. While riding, nothing is static (except the skiers you pass :D ) Strength is definitely necessary, but if your muscles can't fire when needed to keep edge pressure on rough terrain, or to regain balance when your edge starts to break loose, it won't matter how big those quads are. http://www.exrx.net has some examples.