exactly - the best way is just not to memorise an essay - know your material inside and out. heres my suggestion, take it or leave it:
-have a look at the prepared essay you spoke of. go through and list all the major points you make, all the techniques etc. then, see what connections can be made between the points (ie. how they link together). if you study this, then you'll have a thorough knowledge of all the major points and how they fit together. this then, is far easier to use to write an answer for the question asked then a prepared essay is to manipulate to the question.