Look over the entire exam so that you know all the tasks that lie before you.
Order the questions according to what you know best. Start with the one you feel you can answer best.
Set a time limit for each question, and stick to it.
Make an outline to structure your answer. Follow your outline as you write.
Don't look over your answer once you're done; instead, move on to the next question. If you have time left over, then go over your answers.
Remember to pay more attention to content than to style. Your teacher understands the tight constraints of an essay exam and will make allowances.