Some people memorise by writing, others by reading it over and over again. Personally, I read it out loud to myself.
As with slyhunter, I also just remembered key points (ie. topic sentences or phrases), quotes, techniques - and then reassembled it all into an essay in the exams. Depending on the essay and the time you have, you may drop/add things along the way.
read new sentence. close eyes. recite sentence aloud. open eyes. recite all of the essay up to that sentence aloud. repeat until entire essay is memorised while walking around the house and annoying the bejesus out of the parents.
that's how I did it anyway. but as pointed out above, memorising an essay is not the only way to get full marks, nor will it always get you full marks.
Write it out three times a day starting from the day you get your assessment notification and only look at your notes when you absolutely need to. Once you're confident, start reciting it in your head when you're not busy, for example, when you're sitting on the bus. Eventually you'll be able to recite it without even thinking.