Be prepared emotionally and physically, as well as intellectually - Be prepared to do your best.
Prepare your brain for optimum performance by keeping your physical resources well maintained:
Make sure you are calm, relaxed and confident. You have worked hard, and there's no use in being nervous now. The more calm you are, the better you will perform.
Stay away from others right before the test.
Anxiety is highly contagious. It is best to focus on what you know rather than on what you don't know. Reinforce your strengths and confine your weaknesses. For this reason it is also best not to study new material the night before a test.
Arrive at the exam room early
Give yourself enough time to select a seat and calm down before the test papers are distributed. Select a seat where the lighting is best (frequently near the front of the room or near the windows) and where your view of other students will be minimized.
Remember to bring your student index and more than one pen / pencil. Dress warmly and comfortably (and in layers, so you can put on your sweater if you're cold).
When you feel yourself getting nervous during the exam, concentrate on breathing and staying calm.
Here is a short video that can help you practise your breathing: