Decide the stance you're going to take in the essay. You may need to figure out if you support or disagree with the essay topic.
Organize your essay before you dive into writing it. You'll only have a short time to complete the essay, but it's important to take the first few minutes to plan out your writing. Make an outline containing information that you'll need to include in the introduction, body and conclusion of your essay.
Train your ears to hear sentence errors in order to prepare for the multiple-choice questions of the test. You can do this by reading well-written articles. For example, reading the New York Times on a daily basis will help you to understand good sentence structure, and to recognize bad sentence structure when you're faced with it.
Read the question, making sure not to look at the multiple-choice answer options. Try to find out what is wrong with the sentence and then look through the answers to match your deduction.