The majority of anxiety stems from the unknown, the what ifs that float around in your head. The more you know, the fewer what ifs to be had. Know the format of the GMAT inside and out. Know which sections come first, second, and third. Know the rules regarding scratch paper and bathroom breaks. Most importantly, study. Know the material that will be tested and how it will be tested. If you are a master of the information covered on the GMAT, then anything the test throws at you, you will be able to handle. If you're confident in your abilities, you'll be able to fight off those jitters.
2. Once you've prepared, prepare even more
Sure, you've taken a few computer practice tests and even slept with the Official Guides under your pillow (much to the chagrin of your significant other). But you can still do more. Figure out where the test center is and make a trial run. Know what you're going to wear (yes, I'm actually serious about this). Choose comfortable clothes and bring layers; you don't want to be too cold or too hot during the test.
Close your eyes and visualize your favorite place. Take deep breaths. Go for a jog. Listen to your favorite Gregorian chants. Get a massage. In the days leading up to the GMAT, relax as much as you can. This isn't to say you stop studying but don't focus exclusively on the test. Also, remember that the GMAT is not a test you can cram for; you will know the vast majority of the material well before those final days and whatever you try to sneak in right at the end is not really going to matter. Do some practice problems and review your time management strategy, but do not go nuts. Do make sure to get plenty of rest and good food. But go out, enjoy life, and leave that Official Guide under your pillow.
4. Indulge a bit
Get as much rest as you can while you gear up for the GMAT. You may not have slept eight hours in a night since you were three years old but, guess what? Now's the time to know what it's like to wake without the alarm going off (or to go to bed before The Daily Show). Also, now is the time to make sure you're taking your mother's advice: eat your vegetables and take your vitamins. You want to be in top physical shape.
5. Have a post-GMAT plan
Look, you've got a rough day ahead of you. No one is denying that. So give yourself something to look forward to, something positive you can think about for two seconds when you're bogged down trying to figure out if you're dealing with a compound subject. Plan to meet a friend for dinner, have tickets to that evening's ballgame, or book a manicure for an hour after you're done. You're almost at the finish line!
6. If things do start to go south, keep your head up
One of the worst things about test anxiety is that it can be self-perpetuating. What happens if you get in there and flub the first question? Your confidence is going to start going down the tubes. Don't let it. This is something you can control. The GMAT is designed to be hard for everyone; don't beat yourself up if you can't answer a question. First of all, you don't have time to do it and, secondly, what's past is past. You have to let it go and move on to the next problem.
7. Remember, it's not the end of the world (or your bschool prospects)
The GMAT is just a test. And you can always take it again. It's also just one part of your application to business school; it won't make you or break you. Yes, you want to do well. And you should do well. But if you don't, it really and truly won't ruin your life.
So take a couple of deep breaths. Shake out your hands and do a couple of neck stretches. Close your eyes for a few seconds and just relax. And if all else fails, remember that Dick looks pretty funny walking at three miles/hour in his tighty-whities.