Using body mass to change direction
Many intermediates try to change direction by turning their entire body. First they lean into the turn. Then, their uphill hand drops behind them, out of their field of vision. Steering with your shoulders, torso and hips creates a delayed reaction: Your upper body twists harder and faster than your skis actually turn. This is ineffective; by the end of each turn, your shoulders will be facing away from your direction of travel.
Steer with your legs and feet
Face where you're going and keep both hands where you can see them. If you lose sight of your inside hand, you've rotated into the turn. If you lose sight of your outside hand, you've counterrotated away from the turn. Both are problematic.
Turn from the bottom up: Feel your skis roll onto their edges first.
Guide both skis with your feet and tilt your lower legs.
As you steer past the fall line with your thighs, feel your knees driving into the turn.
Complete the turn with your legs. If you're looking where you're going and both hands are in view (think elbows in front of ribs), then you haven't rotated and you're correctly facing the direction of travel.