Good sleep before the test is key.
If you want to have a bit of an advantage, get a pre-workout supplement called "Craze". It's a low-caffeine grape-drink powder that stimulates you but doesn't "buzz" you. It's main effect in lifting heavy weight is that it clears your mind and helps you focus on what you're doing. Because this is how it works, it also just so happens to be great for taking tests because you need to focus as much when taking a test.
Make sure that you take it 30 to 45 minutes BEFORE the tests starts though because it's not like traditional energy drinks such as Monster, Red Bull, or those 5 Hour Energy things. It takes a bit to begin working because of the lack of caffeine.