Very rarely do I see a good player swing the club past the horizontal position at the top of the swing. Anything longer results in a loss of power and inconsistent contact with the ball. Basically there are two main causes of overswinging: 1. Losing control of your grip on the club at the top, and 2. losing the width or radius of your backswing arc as a result of the right arm not working correctly.
The sight of a severely bent left arm at the top would appear to be the cause of a typical overswing, but such is not the case. The bent left arm is actually the result of an out-of-position right arm. The right arm controls the width of the swing. If the angle between the upper and lower arm is less the 90 degrees and overswing will result.
Such overswings are often accompanied by a lack of effective coiling in the torso. So the key to shortening your swing is to first establish the width of your backswing arc.
To achieve the width that characterizes a proper backswing the right elbow must work in a way that as it folds it forms a right angle, or "L" shape at the top. Your left are will be comfortably straight, not tense with your right elbow positioned at 90 degrees.
With the help of the these keys you will soon learn to combine a fully coiled upper body (90 degree shoulder turn) with a wider swing arc. The results will be more consistent contact and more powerful shots.