There are four key elements that are vital to writing a good essay: argumentation, clarity, fully explained points, and concision. Before you begin writing an essay, you must know exactly what you are going to argue and how your argument relates to the question asked. This will require you, in planning time, to pull apart the wording of question and determine whether there are any key words which need defining before you can begin your answer. Once you have an idea of your overarching argument, the next step is gaining clarity. This requires you to be clear of exactly what you want to say and the order in which you plan to say it. The body of the essay will require you to make one well-explained point per paragraph. My advice here is not to rely on examples to explain your argument. Examples must be used to illustrate a point that you have already made clear; statistics and facts are not arguments. Finally, you will need to ensure that your writing is concise - a longer essay is not necessarily a better one. It is more important to convey your argument in a succinct way than to use extraneous fancy language just to show that you have a large vocabulary (often it is quite obvious when students are doing this). Showing the reader that you have control over your own writing is vital, and examiners never like reading waffle.