The SAT Writing Section's Essay question is always the first question of the test.  You get 25 minutes to outline, write, and edit an essay on an unknown topic.  The instructions they give you are always the same, and so is the rubric; basically, they evaluate your organization, style, and content (kind of).  Here are some prompts from recent tests.

This test requires a specific type of essay, one that is much easier than what you write for English class.  It does not call for intelligent commentary or an insightful conclusion.  Above all else, it rewards formulaic responses that follow a specific structure.  In other words, the graders care more about how you write than what you write.  

ORGANIZATION & CONTENT

If your essay is organized like this, the graders will know where to look for each part of your argument (like your thesis or different examples).  The quickest way to be organized is to outline your essay before writing it.  Outlining an SAT essay is simple because we already know the list of things that the graders want.  Because they reward formulaic essays, all you've got to do is just fill out their form:

Introduction-->Examples-->Conclusion

STYLE

Every point you make should be as clear and specific as possible.  Make it easy on the graders; they're stuck inside all day with only a minute or two to grade each essay before rushing to the next one.

Vocabulary

Use at least 5-10 good vocab words in your essay to impress the graders.  Make sure to use them correctly; misusing a word counts against you.

Length

Fill each paragraph with at least 3-5 sentences.  This is more important than you may think.  A 2005 study by Dr. Les Perelman (director of MIT's undergraduate writing program) revealed an astounding correlation between an essay's length and it's score. "If you just graded them based on length without ever reading them, you'd be right over 90 percent of the time," says Dr. Perelman in a NY Times article on the study.

Specificity

On an SAT essay, sounding specific is more important than being factual.  In other words, your facts don't have to be 100% right, just not 100% wrong. 

This distinction is meaningful here: the graders will forgive you if your facts are a little off, but not if you're obviously making stuff up.  But since they do overlook minor factual errors, it's possible to get away with embellishments.  Stay within reason; the graders are rushed, but they're not stupid.  The following quotes from historical figures are all fictional.  Two of them are within reason, and two of them are not.

"A government's magnanimty subsists on the liberty of its people." Thomas Jefferson

"From Boston to Los Angeles, these United States are our birthright." George Washington

"What treachery is bolder than Freedom's oppressive decision?" William Shakespeare

"Either you planting imperialist seeds in this New World, or you just hoeing." Kri$ Colombus

Of course, we're not suggesting that you lie on your SAT essay.  We're just pointing out that they don't care if you're completely accurate or not.