Remember that some nervousness is normal. It can increase your alertness and improve your performance.
Don't be intimidated by the examiner. He is merely a person doing a job. He is not a god and he will not be judging you as a person. He doesn't want you to fail your test and he will not try to trick you at any stage.
Don't try to interact too much with the examiner. Silence is normal in the test car, and too much chit-chat could be distracting.
Listen carefully to the examiner's instructions. Ask for repetition / clarification if necessary and act in good time.
Bear in mind that the examiner just wants to see how you would normally drive - nothing you don't already know!
Explain your decisions to the examiner if you feel you have done something that could be misjudged.
If you come across a new situation, don't rush in. Stay calm and assess things carefully before you proceed. Be prepared to change your decision if necessary.
Take a deep breath and exhale slowly if you feel you are getting a bit tense at any point in the test.
Don't give up if you feel you have failed the test. You may be mistaken, so don't drop your concentration levels and risk commit errors that do cause you to fail.