over swinging the driver

[punchy]

I have a tendency to over swing with the driver. That is, I make a full shoulder turn, but then keep twisting. When I do start the downswing, I get the nasty outside to in turn and the result is a weak drive that either fades or flat out slices. I have to figure out a way to stop my backswing much earlier. Does anyone have any tips/drills I could use to stop my backswing earlier?

Thanks

Punchy