A great tennis website and coach is FeelTennis.net
I have found this great video by Tomaz about the "Universal Swing" and I would like to share it with you.
Hope this helps!
Here is what Tomaz has to say
"I believe that this week's video will be of immense help to many of you that struggle with finding effortless power on your groundstrokes in your tennis game.
You will learn the physics behind the so-called Universal Swing that applies to all groundstrokes and why this helps you hit more effortlessly.
I will also show you a simple drill I use almost daily now with my students that helps them feel the power of the Universal Swing.