The most important factor before taking an exam is that you should be prepared. If you feel prepared you will be able to relax more effectively on the day of the exam.
Be Prepared
Create study checklists and try to work out all of the course material that you may be tested on - list notes, formulas and ideas. This checklist will enable you to break your studying into organized, manageable chunks and hopefully make revision less stressful.
Create summary notes and "maps"
Briefly map out the important ideas of the course and make summary notes.
Creativity and a visual framework will help you recall these ideas.
Tape Yourself
Record your notes and significant portions of text on audio tapes so you can review course material with a walk-man - this way you can do other things around the house/outside whilst revising.
Flashcards
Flashcards are basically cards which have key points on which you think are important (Definitions, formulas, or lists), put topics on one side of the card, answers on the other. Flashcards will enable you to test your ability to not only recognize important information, but also your ability to retrieve information from scratch.
Remember not to take them into the exam room!