1. If you have played against the opponent before, you can make a game plan. It doesnt have to be every detailed. For example:
I will hit more short angles, etc
2. Try to get a good sleep and eat proper food, if you do this, you have one thing less to worry about.
3. Engage in some activity to keep your mind off the match e.g. watch TV or Movie (action or comedy is prefered), read something (book, internet), play a PC game etc.
4. Before a match listen to some music, these days I listen to Daft Punk.
5. Most importantly, if you cant keep your mind off the match tell yourself, in fact say it louder or shout "
I will dig deep ! I will fight for each point even if I am 40-love down, I will play one point at a time and dont think about the previous or next point and I will give my 100%
6. DURING the match keep on reminding to yourself about your technique,
For example: move the feet and keep the eyes on the ball
7. Pretend to be someone else e.g. if you are a fan of Federer, imagine yourself to be Fed when you play the match
Act like him and you might overcome the anxiety.... Role play can actually work !