Where many people think they are creating a full shoulder turn at the top of the backswing, they are actually tilting their upper body. This awkward position will usually result in them cutting across the ball through impact creating that horrid slice shot.
If this description matches you, then try this cane drill from TG’s Dan Frost to create a far more efficient shoulder turn. By strapping a cane to the front of your chest with Velcro straps, you can get an idea of the angle of your shoulders in the backswing and easily correct your faults.
By keeping the left side of the cane as level as possible in the backswing, you can get more turn with the upper body, a more level swing and will be able to bring the club down on a nice, neutral swing path.