Writing a good essay requires synthesis of material that cannot be done in the 20-30 minutes you have during the exam. In the days before the exam, you should:
Anticipate test questions. Look at the question from the last exam. Did the question ask you to apply a theory to historical or contemporary events? Did you have to compare/contrast theories? Did you have to prove an argument? Imagine yourself in the role of the instructor--what did the instructor emphasize? What are the big ideas in the course?
Practice writing. You may decide to write a summary of each theory you have been discussing, or a short description of the historical or contemporary events you've been studying. Focus on clarity, conciseness, and understanding the differences between the theories.
Memorize key events, facts, and names. You will have to support your argument with evidence, and this may involve memorizing some key events, or the names of theorists, etc.