I would agree with Doh, the most common reasons are the over-bent left arm (often itself caused by the arms coming away from the torso way too early) and certainly as soon as I kept my right knee flexed throughout the backswing, things were dramaticaly better. My new pro has given me a completely different take-away move (sorry to those that don't like the phrase) that max's out the width, keeps the face a bit closed and changes the moment of wrist cock to much later....it's v.v. good. When I'm well, I will try to post some piccys.
In the meantime, I'd recommend some half swings with a headcover under your left armpit (r hand player).