Learn about the essay questions used in the AWA exam. Most AWA practice books will show you the majority of the actual questions. You can find the books at most major bookstores or libraries. You can also find essay questions at the GMAT's website (mba.com) but you will need to create an account.
Practice writing essays. Practice is the key to becoming a better essay writer. Time yourself so you only spend 30 minutes writing the essay. Have a teacher rate the paper and critique it for you. Write with little humor or any personal individuality since your main rater will be the E-rater. It will not appreciate your sense of humor and will even knock down your score.
Read the directions and memorize them ahead of time from your AWA practice books for your essay. Knowing the directions ahead of time will help you save time during the actual essay.
Get a good night's sleep the night before the test to ensure you are well rested.
Eat a good breakfast the morning of the test to have energy and avoid being hungry.