The essay assignment probably contains more information than you think. Don't skip past it in your frenzy to answer the question. Consider how the quote relates to the question. Decide if you agree with the quote. If not, on what grounds do you disagree?
Now is not the time for the "can't we all just get along?" mentality. SAT essay questions are written in a manner that begs you to take a stance. Never say you agree with both sides. Before the scorer finishes the first paragraph of your essay, he should know exactly which side of the issue you support.
After you decide your stance on the issue, don't just start writing. Take a moment to sketch out a 4-paragraph plan. Paragraph 1 will be your introduction. Paragraphs 2 and 3 will provide examples that support your argument. Paragraph 4 should tie up all your loose ends. Having a plan will give your writing direction.
Effective writing requires you to prove your point. The best way to offer proof is by example. Choose a historical or literary figure that illustrates your stance. It's a good idea to pick a few examples beforehand and thoroughly familiarize yourself with them. For instance, you can tie either President Obama or former President Bush to almost any prompt hurled your way.
If you really want to score big, consider playing the devil's advocate. In the third paragraph, explain why someone might disagree with your point of view. After the brief explanation, provide another example that dismantles the opposing argument. If you can pull this off, you're sure to see your score go through the roof.
No matter how fancy your essay is, writing off-topic will cause your score to plummet. As you write each sentence, check to make sure it relates to the question and furthers your stance. If the sentence fails to relate, get rid of it.