It is important to get a handle on test anxiety early - to understand why you are anxious and know how to handle it. Reducing test anxiety must start when you begin studying for a test, not just the day of the test.
Reducing test anxiety starts with preparation. When you first find out that you have a test coming up, make sure to find out exactly what information will be tested and what format the test will be (multiple choice, short answer, etc.). Take time to prepare, and schedule your study so that you learn a little of the material each day. This slow and steady approach helps reduce your anxiety, because you are tackling the information in small chunks.
On the day of the test, there are some ways to prepare before you actually start the exam. First, make sure you eat before the test so you aren't hungry (which can be a distraction during the test). Also, wear clothes that are comfortable and that you feel good wearing. Finally, if you have a choice, sit far away from distractions (windows, doors, people who tap pencils, etc.)