When I am helping a golfer who is having trouble with his swing, I often advise him to practice swinging while standing with his back about 18 inches from a wall. You will find that when you swing back and do not hit the wall, it is virtually impossible to fall into the common error golfers make on the backswing. Your left shoulder will have to stay down and can't roll up and over. The club head will move backward in a rising arc. At the end of the backswing, it will be pointing in the direction of the target, and its face will not be open. However, if you drop the club on the back-swing so that the club is lowered toward the right shoulder, the club head will hit the wall, and this will inform you that you have swung back incorrectly.
Next try facing the wall, spotting the club head about an inch from it. When you practice swinging in this position it will help you to correct errors in the forward part of your swing. It will make you keep the left shoulder down and under. If you don't, again the club head will hit the wall. As you swing facing the wall, you will find that you will use your left hand as a pull-down for power instead of using only your right hand and right shoulder, a fault which makes you hit over the ball. If the left hand is pulling down, the right will follow naturally, adding power. Golf is a two-handed game, but the left hand dominates. The right hand helps it.
from BILLY BURKE The Country Club, Cleveland