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Overdoing it could be the most common mistake I see on the practice field.  It also seems to be heavily misdiagnosed with a fix that can only lead to temporary results.  Best seen as a long backswing, or going past parallel, this can lead to an incredible amount of swing faults.

Unfortunately, this fault is too often associated with the body parts closest to the golf club… the hands, arms and torso.  Fact of the matter is, the golf swing stems from the ground up and this swing fault is no different.

These issues stem from overusing the feet for balance and stability which allows the knees and hips to get out of position.  This problem follows the rest of the link to the club…once the lower body is out of position, the upper body cannot be in the correct position. 
A great drill to fix the flat shoulder plane is to create instability in the trail foot which will force the knee & hip into a stable position.  Use a 1.5″ angle iron, which can be picked up at any home improvement center for less than $10.  Place this underneath your trail foot and make sure that your foot stays on top of the rail, not on either side of the rail.  This is the instability we need for your foot to force your knee & hip to become the stable entity.