Mine is not so much a drill, but a training aid.
One of the most common problems I see is transition from backswing to forward swing. Most people tend to hang back on their trail leg, or try and initiate forward swing with the arms instead of the lower body.
I have had great success with people throwing a 6 pound medicine ball, in the motion of a golf swing.
The weight of the medicine ball, with the concept of throwing it, really helps people feel the transition from backswing to forward swing.