Practice swinging "up the hill" on the side of a driving range, the side of a tee box or anywhere else you can take a slight uphill stance. Swing up the hill on the side of a tee box several times before each of your tee shots during a round to remind yourself of the proper feeling.
Video: How to correct a slice
(Editor's Note: Michael Lamanna, director of instruction at The Phoenician resort in Scottsdale, Ariz., uses the "Up the Hill" drill with his students. He is also the handsome gentleman pictured in the photos.)
