Strategies for Math
For math, the best preparation is to do math. Make sure to keep up with your homework in school, take math AT LEAST through Algebra 2, and try your best. The more math you do, the more comfortable you will be when the SAT arrives.
Of course, there are certain strategies for the exam that are useful:
Estimation
If a particular diagram is drawn to scale (which they always are unless otherwise specified), then use common sense. Get rid of answers that look too small or too big. Try to figure out the relative largeness of a value with respect to the choices if you can't directly solve the problem.
DRAW!
I cannot specify how important this strategy is. It's not only a strategy, it's a necessity. If you find there's a weird geometric figure that you've never seen before and don't know how to tackle (for area, perimeter, etc.), then DRAW LINES! Try to break it into things you know how to work with or try to extend things to make something you're familiar with. Make sure to mark angles, sides, etc. Geometry problems should be thoroughly written all over when you're done.
Plugging in
It's multiple choice! What better way to find which choice is correct than to test them out? For something you can easily tackle, don't bother, as this method is inefficient. But for something that is difficult, then just try to plug in to see what works. At worst, you'll eliminate the blatantly wrong answers, and at best, you'll eliminate all the wrong answers.
Simplification!
Another really important one. If you get a really long expression or quotient, odds are, you're meant to simplify it. Usually just cancelling out a factor or two will magically yield what you're looking for. Oh, so satisfying!
Factoring
Always, always be able to recognize how to factor/unfactor something. Many times those intimidating numbers & operation problems can be solved if you just factor or unfactor. Learn things like the expansion of (a+b)^2 and how to undo its expansion to get this. With practice, this becomes second nature and you gain a LOT of points!
Substitution
Learn to recognize when to plug in one quantity for another. Usually this solves a great number of the harder problems.
For a review of the actual mathematics, the Official SAT Study Guide provides a fairly good refresher. I suggest that almost everyone work through it -- there's always something you never learned/forgot, even if you're in upper level math.