Have you ever walked smelled something and been instantly transported through time and space into a memory so viscerally real, it feels like you are almost hallucinating? The deeply associative power of smell on memory is very real and well-known. Back in college, I used it to pass several tough exams. Here's how:
While studying for an exam (generally at the last minute the night before), I would keep a particular, unique odor around. Basically reading, working through the material, etc. while constantly smelling a particular smell. Then, at the time of the test, I would simply bring that smell with me and voila! Instant recall straight from the basal ganglia.
Over time, I experimented with several different smells and olfactory substances and found some rules-of-thumb:
During the associative phase (i.e. while studying), best to start with the smell, then remove it before you become fully desensitized to it. Then, reintroduce it regularly, say every 5 minutes or so, but never long enough for desensitization to occur.
During recall (i.e. during the exam), the best technique is to start off with the smell (this really puts you into the frame of mind of the test and the subject matter), then hold off completely unless or until you are really stumped or hit an essay question that requires complete immersion in the subject matter. Sometimes I had actually prepared well enough that I didn't even need my scent-boost ;-)
I experimented with several different scents and delivery vectors. The trick is to find something fairly unique, yet pleasant, that won't create suspicion/anxiety/discomfort/etc. for your fellow examinees, and most importantly, something that will retain it's scent over the course of an hour-long or more exam.
Many obvious candidates (e.g. cologne/perfume) fail because they can't easily be 'turned off and on' at will. So, over the course of the day of the exam, you quickly become sensitized to them, and they are useless. You can carry them around in a little bottle to get around this problem, but then it becomes highly distracting to take it out, open it, and sniff it mid-exam.
The best substance I ever found was peanut butter. It is easy to smudge onto your hand or arm or under a fingernail. Its smell is fairly unique and strong, and it will last for hours. Best of all, if it is on your hand, you don't really smell it, and a casual wipe of your nose mid-exam is completely normal.
To this day, whenever I smell cheap peanut butter I begin mumbling arcane biology facts: prophase, metaphase, anaphase, telophase...