Upper-body movements should complement what's going on below.
When you are not in a challenging situation, just let your trunk and arms go along for the ride. As you casually glide or turn, your upper body should be a passive, single unit. Don't bend, twist or turn it too much-excessive movement will only throw you off. Line up your torso with your eyes facing the direction you intend to go. Allow your skis and lower body to do the work.
As you make skidded turns on fairly flat skis, allow your skis and feet to drift sideways to kill speed. Let your upper body tilt to the outside-toward the ski that will soon be downhill. An angle should form near your center, as your hips stay inside and your shoulders tip outside. Such angulation compensates for the skis' drift, keeping you balanced over your outside ski, preventing the skis from slipping out from under you.
In carved turns your upper body, neck and arms must be positioned to deliver pressure to the edge of your outside ski. Your shoulders and hands should stay level with the slope. You should feel strong-but not rigid-from the waist up, especially in the uphill half of your upper body. Your downhill half needs to balance solidly over the top of your thigh bone. Your carving ski is like the rim of a wheel-you're a spoke.