Forget about studying for the rest of the night. Taking one more practice exam or going over flash cards can do you more harm than good if you've been studying hard for weeks. Instead, give yourself a break from studying and other academic pursuits and just pamper yourself.
Exercise or do yoga. Physical activity is a great way to work out your anxiety, and the adrenaline rush you'll experience from getting your heart rate up will relax you once you're back at a resting pace.
Take a long, hot bath and just let your body soak. It's hard to remain all keyed up as your thoughts drift away like soap bubbles.
Watch a funny movie. "How I Got Into College" is appropriate on this occasion, but if you'd rather not think at all about tests, pop in anything that makes you laugh.
Be philosophical. Think about the big picture. Whether you ace your SATs might not mean anything to you ten years from now. It's true that a great SAT score can open the door to scholarships, admiration from peers and adults and admission to a prestigious school--but it's also true that this is not the only path to happiness or success.
Give yourself a pep talk. Remind yourself of all your hard work, and tell yourself you're going to do a great job. Confidence will go a long way once you're sitting in front of the SAT test. It might mean the difference between nervously skipping a question and acing it--so convince yourself that you're a superstar.