Make a list of potential topics that the essay questions could fall in and prepare a list of bullet points for each topic. This makes it easier to remember, and when you are in the exam, you can jot down the bullet points to form an essay plan. It means that all of the points you want to be covered are included and you get more marks when you expand on each point.
Another tip is to do practice essays. It gives you an idea of what the exam is like and how to complete it in the given time. You could use the bullet point technique when doing a practice essay. I don't know about your school but some teachers are happy to mark practice essays and give feedback, which can be very useful!