How can I clear my mind?

Meditation, listen to classical music, read a book.

Good luck!

You don't actually clear you mind, you'll be missing the ball. You set your mind into the game. From a lot of interviews I hear from athletes, they imagine how they will be playing the game prior the actual action that the athlete will be taking so that the athlete will know the possible outcome to win the game. For example, a golf player is about to take his/her shot. The player will then imagine how the shot will be like at a certain angle and power then if the player is ready, then he/she will take the shot. At a professional level, the shot is more likely the same as imagined. So you actually focus into the game! Good luck! =)

A movie often works well for me.

Breathing techniques perhaps.

Relax yourself and do something you enjoy, while resting.