For univeristy/college essay type exams the trick is to steal the paper/booklets they all seem to use in a previous exam (easy enough, just ask for another one to write a better copy). Then go home and write out your notes for the probable essay questions in a format that looks like paragraphs (in a barely legible handwriting helps).
Find a way to smuggle it in without folding it, either the whole booklet or just a few pages. To do this you can put it under your shirt being held in place by your belt, wear a jumper to hide the slightly irregular shapes.
when you sit down for the exam the first thing you do is pull the staple out, letting the 20 or so pages cover the table, work on a couple of the questions for a while and then let a few pages fall to the ground.
Bend down and collect them and remove your notes without being seen by the supervisor, place your notes with your other loose sheets which have the same format, line spacing and all that
.
Now they've got no way to prove you cheated and you can openly work on it.
I havent actually used this because my goal at university is to learn
, but my friends have used (and some still are) to great success. It gets alot harder to do if your college uses different colored paper for different exams, but academics seem to love everything being uniform. We have a few other effective methods, but i'd rather they didnt become mainstream till AFTER we finish.