Make sure to eat a good breakfast--as obvious as it is, it really helps. If I don't eat enough, I'm only thinking about how hungry I am instead of the test.
Also, again, an obvious one, but get a lot of sleep. A lot. I try to get extra sleep before a test because my usual 7-8 hours of sleep isn't enough--especially if the test is in the morning, I'll find myself yawning and not concentrating.
Finally, be prepared. I'm not saying that you need to know the material better, but even if you're already an expert at it, the more you review it, the more comfortable you will be with it. I tend to blank out during a test sometimes, and then afterwards, think to myself, "I knew that!" The more I study, even if I already know the material, the less this seems to happen. It will just come more naturally to you.