TIP 1 - Don't stress too much. Stress makes you lose concentration.
TIP 2 - Get ALOT of sleep. Something between 8-10 hours. Not to much though, or else you will be groggy
TIP 3 - Don't think that studying will help. The idea of the SAT and PSAT is to see what you KNOW. Knowledge is not something you can learn in a day. Knowledge comes from school and class and broing stuff like that.
TIP 4 - Try a few practice problems the night before. They can be found all over the web. Don't worry about taking a whole practice test, it is a waste of time.
TIP 5 - Remember a calculator, pencils, and clothing.
TIP 6 - Eat a hearty breakfast. When i took the SAT and PSAT, i got a good breakfsat of eggs, grits, toast, and milk beforehand. It gives you a nice, settled feeling.
Lastly, TIP 7 - Relax, take a deep breath, and realize that you are better than every other person in the room.