In order to deliver the golf club powerfully into the back of the golf ball, you must maintain a firm base with your lower body and create a powerful backswing coil. This coil results by turning the upper body against the resistance of the lower body. Good players facilitate the creation of coil by maintaining the gap between the knees on the backswing (right). They unleash the energy stored in the coil by closing the gap on the downswing.
A lot of amateurs -- slicers especially -- use too much upper body on their downswing. To work on the correct motion, place a Swiss ball against a wall. As you practice making your downswing, use your hips to compress the ball against the wall.