Read the instructions twice, and read the entire test through before starting.
Essay questions: create an outline for yourself, and begin your answer with a summary sentence.
Short answer questions: Answer only what is asked, and if proper terminology evades you, use your own words.
Multiple choice questions: read all the answers, and delete the most obvious one first (rely on your first responses with these questions; beware of tricky words like "only," "most" or "usually").
Don't rush; be aware of the time, and if you're running low, answer the questions of which you're most confident.
Recheck your answers only when you're finished...and only if you aren't anxious.
Stay focused on the test; tell yourself to be nervous after the test.
Don't think about how fast anyone else is going, or wonder about their performance.
Relax; take deep breaths and loosen your muscles when you feel tense.
If you really need to take a break, get up--break your pencil lead and go sharpen it; ask to go to the bathroom or ask the teacher a question.
A little jarring, but this video helps take your mind off the test and also imparts a reminder to your brain to literally "disconnect" from anxiety and free up resources to focus on the test.
So: who just made it through a test and managed their anxiety?
Now that you've made through the test, treat yourself to a reward! Don't jump straight back into studying; your brain deserves a break, too.