Originally Posted by 10isfreak
Not that this is false, but your swing path has a greater influence on the ball's flying increment than on its spin.
If you want more spin, without changing anything else, close your racket face a bit. The idea is that you'll get to make contact above the center of the ball with a closed face, meaning that the upper edge will get accelerated forward. As you swing a lot faster forward than upward -- I've rarely seen pros going beyond 30 degrees of swing path prior contact, including Nadal on clay -- you'll get more spin that way.
I do not agree. If you close your racquet face without changing anything else you'll just hit the ball lower over the net, or perhaps into the net. This is my experience as a player, and also what I would deduce from physics (I have an Masters in Mechanical Engineering).
If you want more topspin you need a greater vertical component in your swing. Open or close the racquet face to get the appropriate height over the net.