I generally disapprove of memorising essays, but if you're almost certain about the question you'll get, here's the method.
Don't memorise the intro/conclusion - this will stop any temptation not to answer the question. Then number all the main body paragraphs, and write the numbers (e.g. 1-5) down the side of an A4 page
Then next to each para write three dot points that summarise the content of your paragraph. For example you may write a quote, two techniques and what this says about a journey in dot point form.
So then you've got it all written down in point form on one page - this will help people with photographic memories.
Now to actually memorise it. Have the paper a foot away from you. Then go through the paragraphs in your head. For example think para1, okay, what am i talking about? Oh yeah! 3 points... I remmeber the first one was about x...oh yeah and the quote was y!
Keep going over it in your head until you are pretty confident you can recall it all. Then as a final test, write it!
And there, essay memorisation in 1.5 hrs.