One of the most important things is to remember to have a clear structure of an introduction, main body and conclusion. You also need to make sure you are consistently referring and relating back to the questions. This allows your examiner to know where you want to go with the essay and helps you when writing under pressure to avoid repeats of information. I would recommend thinking of five to six points for your main body, half for and half against. You should lay out the points you will make in your introduction so the examiner can clearly see that by the end you have achieved your objectives. When writing your main body use the PEA (Point, evidence analysis) structure to form each paragraph. It is very important to analyse each point to get the higher grades, these exams are not just about what you know, but how you can apply the information to draw comparisons and conclusions. Even if you don't finish all of your planned points make sure to leave ten minutes at the end to write a conclusion. It is here you should acknowledge the strengths of all the arguments but support one side to show your ability to think critically and construct a historical argument using and testing the evidence available.