I mentioned this in a previous post on keeping the back facing the target for as long as possible in the downswing to help hit inside-out, but I felt I needed to emphasize the first part of this equation. You must create separation between the lower and upper body on the downswing (i.e., often called X-factor).
Once the back is facing the target, begin shifting weight to the left leg and rotating around it, while the back stays facing the target and the arms begin dropping. This move is discussed extensively by Ben Hogan in his milestone book (also by Jim McClean), and it's so critical to creating a powerful downswing that's driven by the lower body.
What really makes this work is the separation between shoulder and hip position. The shoulders should only catch up to the hips in the follow through. If they catch up early--or worse, outrace them--be ready to look for your ball in right field, because you'll likely get a slice.
If you get a push or slight slice, you simply need to close the stance (i.e., get the face closed to the path) to get that powerful draw working.
Another way to think of it: the shoulders mostly move first on the backswing and the hips mostly move first on the downswing. Voila! X-factor!