Don't.
Memorise the ideas of your essays, and the quotes you may need. Unless its an assessment where you are given the question ahead of the exam, its probably not the best idea to memorise an essay. Have a good understanding of your text and you should be able to answer any question they can give you, from that point all thats left is bringing in the quotes, which is the important thing to memorise.
So instead of memorising essays, write a variety of practice essays instead, and just try to remember the general gist of them. Then just memorise your quotes and you should be much more prepared for a blind questions.
at least this is what me and most of top 10 in eng at my school did.