The exam begins before you even enter the room. If necessary, you still have a few minutes left to revise. The average person has a short-term memory of around 40 minutes, so now's the time to commit to mind the trivial facts you keep forgetting - dates, numbers, and other irritating rote-learned data. You may want to write them down when the exam begins. Writing things on your own body before the exam is not permitted, but after the exam starts, anything goes. (The rules on actual tattoos are unclear. You may find scrap paper less painful.) A copy of those elusive formulae in written form may prove useful before you go in, but taking it with you will spell academic death.
At the start, the rules of the exam will be read out, but bad acoustics and/or moderators' accents usually mean it's a good idea to read through the rules yourself. Most of them will be generic (don't cheat, don't forget to write your name on the paper, etcetera,) but make sure you know which questions you have to answer. Surely there can be nothing worse than toiling for twenty minutes on something which is essentially worthless. If you have to choose your questions, choose early and choose wisely. Read through the questions and think about how you would answer them. Just because a question starts easily, it doesn't mean it's easy all the way through. Be sure to analyse the relative easinesses in sufficient depth before beginning. Of course, if you change your mind after starting, you don't have to omit the possibility of dumping the question and starting again with an easier one. But it's best not to find yourself in such a situation.
Timing is of paramount importance during the exam, as the flow of time accelerates out of proportion once you are sat down, so the entire duration of the exam can pass in only a few minutes. Conversely, the five minutes spent standing outside beforehand last for about two hours. In any exam, and certainly in a longer exam, it's a good idea to plan how long to spend on each question. Remember that you can tackle questions in any order, so you can dash through the easy ones and rack up plenty of marks at the start. Even so, if you get stuck part way through a question, there is no point in staring blankly at it for quarter of an hour. Perhaps things will seem much clearer after you've done another question instead. Keep a careful eye on the time, and alter your speed accordingly. If your writing becomes barely legible toward the end, so be it. Barely legible is a world away from illegible.
The exam ends either when you have finished, or when you have run out of time. Try for the first one. If you have a few minutes left (ideally, you shouldn't still have several hours to wait) then put them to good use. Check through your work. If this is too boring, try guessing how many marks you'll get. If this is too depressing, try counting things. There are a lot of bricks in the opposite wall. They could keep your mind occupied. Resist the temptation to stare idiotically around, as this may be construed as an attempt to signal to your fellow students. If you have not finished by the end, then of course you will have to stop. Try to bring your question to a successful conclusion, or at any rate a conclusion of sorts. Scribble down anything you've failed to include as an afterthought, as it could still carry some marks. Bear in mind that the moderators frown upon those that carry on writing more than a few seconds after being told to stop.
When dismissed, walk quietly out of the room. This is done by moving each foot in front of the other in turn. Try not to fall over; the semicircular canals in your ears should give you some indication of how you are doing.