The "Restrung in 24 Hours" DVD by Tennis Guru is AWESOME (as evidenced by the long thread about it). He shows you a different, better way to hit the ball. He shows you the correct grip and stroke pattern for the forehand, backhand (one and two handed), slice, volley (forehand and backhand), overhead, and serve.
It's short but full of excellent content. I like to watch it before I play. The DVD also comes with a book. It shows you the secrets of high level strokes.
On a side note, if you are interested in sources other than DVDs, I know of two EXCELLENT sources. Hitechtennis.com and tennisplayer.net are unreal. They have amazing videos and instruction. Hitechtennis.com has really beautiful slow motion videos. Tennisplayer.net has an extremely large library of videos.
I subscribe to both and they are a tremendous help to my game. Whatever you do, be sure to subscribe to hitechtennis.com if you want high level shots.