Don't go at a speed that you can't handle. This leaves too much room for making unnecessary mistakes. Every time a new question appears on the screen, look away from the computer screen for a second and take a deep breath. This reduces eye and neck strain, as well as provides your brain with oxygen.
On test day, take advantage of the breaks! Use that time to go to the bathroom, stretch, move around, and eat something. And don't forget to splash cold water on your face. These activities will maintain your energy level as much as possible and keep you fresh for the test. You will also stay relaxed. This is important because if you are not relaxed, you won't be able to do what you want.
And don't overdo it during the last week. Make sure that you don't study on the day before the test. Assuming that you studied and prepared properly for the GMAT, then all the information is already in your head. You just need to execute. The best way to do that is to relax and stay in control.