Practice taking the test under conditions that are similar to the ones you will have on testing day. Use a pencil, lined paper and a timer. Practice this several times.
Think before your write. When you first see the question, stop, gather your thoughts, and form an opinion of the given prompt. Then, think of how you will support your opinion.
Create an outline for your essay. This will help guide you through your writing. You can write your outline in your test book, but be sure to put your essay on the lined paper provided.
Write your introductory paragraph carefully and superbly. This is the paragraph that will make the first impression. Scorers will form an opinion about your writing after they read this first paragraph. Use the first paragraph to state your opinion clearly and concisely.
Use appropriate grammar, punctuation and spelling. You should brush up on your grammar in advance of the test by reviewing simple grammar and punctuation rules.
Include SAT words in your essay only if you know what they mean and you know how to use them appropriately. You can learn these words in advance by purchasing one of the books which lists the most common SAT words. Study their meanings and practice using them in sentences.
Find a good writer to coach you and critique your essays. Your parents or someone in your family may be skilled in writing. You may also want to hire a tutor. English majors at local colleges may be interested in tutoring you for a reasonable price.