You can normally guess what topics essay questions will be based on, so I used to make essay plans and do mock essays. I would ask the teacher for feedback for mock essays, and then condense those essays into bullet points concerning key facts and points. I think for me, repetition is the key, so I would rewrite those plans and practise questions and then it gets stuck in my head.
Some people like to use colours or pictures to help them remember things. Or make a story/song, depending on how you normally remember things.