Honestly, the best thing you can do is remind yourself that your GMAT score will not have a big impact on your life and that it's not really that big a deal. Do not pressure yourself on this - take a deep breath, and try to remain calm. Also, don't eat any sugar or drink any caffeine a few days before the test. Both of those can contribute to your anxiety.
Just keep doing practice questions, and focus on remembering math formulas, grammar rules, etc., and forget about the rest. If you know it all cold, you can relax more.