As others have stated, take the practice tests - the more prepared before hand, the more prepared you'll feel the day of and be better able to focus during the test. Be sure you get enough sleep in the days leading up to the test - try to stay stress free and eat healthy, drink plenty of water - this way you'll be feeling good the day of the test and not be battling fatigue.
Another tool I stumbled upon looking for things to help my sister with home schooling her kids is the following site:
The Kahn Academy:
http://www.khanacademy.org/Scroll down near the bottom of the page and there is a whole series of videos where they review every question on a standard SAT test and how to arrive at the solution. The whole site is free - several teacher friends of mine highly recommend it as a great supplemental tutorial.
Good luck to you!