First of all, be sure you've studied properly:
It sounds like a no-brainer, but if you're sure of the information, you'll have less reason to be worried.
Get enough sleep the night before the exam:
Your memory recall will be much better if you've had enough rest. In a scientific study, people who got enough sleep before taking a math exam did better than those who stayed up all night studying.
Listen closely to any instructions:
As the teacher hands out the test, be sure you know what's expected.
Go through the test as a first step:
Once you received the exam, go over the entire test to check out how long it is. In addition try to look at all the questions that you are expected to complete.
This will let you estimate how much time you have for each section.
If something seems unclear before you start, don't panic: ask the examiner for help and try to stay focused.
Focus on addressing each question individually:
While you are taking the test and seem to be unsure about an answer, don't obsess over it. Instead, solve it the best way you can or sip the question and come back for it after you've answered some of the other questions to give you self-confidence.
If you realize that you are starting to get too nervous, you should use the opportunity and take a short break. As we all know, it can be easy to forget things we know well. Reacting nervously to a question, is much worse than loosing 5 minutes regarding a short break.
Despite most of the teachers will let you hand in a test early, it is usually a good idea to spend some extra time going over your exam. You can also add details that you may not have thought of.