The point of a written examination is to assess two distinct aspects of your mind: your knowledge, and your ability to adhere to completely arbitrary instructions. To instil a sense of order into the examination room, everyone in it must know and adhere to many rules. To a great extent, your knowledge is predetermined by your previous revision habits. The rest of your grade relies on your conduct during the exam.
The exam actually begins before you even enter the room. Whether you make something useful of the time or succeed only in destroying your composure is a crucial stage in the test. 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. A copy of those elusive formulae in written form may prove useful before you go in, but taking it into the exam with you will spell academic death.
At the start, the rules of the exam will usually be read out, but bad acoustics and/or moderators' accents usually mean that 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, etc... but make sure you know which questions you have to answer. Surely there can be nothing worse than toiling for 20 minutes on something 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 easiness in sufficient depth before beginning. Of course, if you change your mind after starting, you don't have to reject 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 sit down, so the entire duration of the exam can pass in what appears to be only a few minutes. Conversely, the five minutes spent standing outside beforehand may seem to last for about two hours. In any exam, and certainly in a longer one, 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 a quarter of an hour. Perhaps things will seem 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 towards 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, 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 some sort. Scribble down anything that you've failed to include as an afterthought, as it could still carry some marks. Bear in mind that the moderators frown upon those who carry on writing more than a few seconds after being told to stop.
When dismissed, walk quietly out of the room.
Long essay questions pose their own particular challenge. Plan your answer. Spend a few minutes drawing one at the start. If you think you may be running out of time write a conclusion, and answer the original question directly. Then, using bullet points, list enough other stuff to justify your conclusion.
Cramming before the exam is not necessarily a good idea, particularly in subjects that emphasize understanding rather than a simple regurgitation of facts. The chance that you will understand anything better in five minutes of intense squinting just before an exam begins is a slim one (although you might remember the odd fact or formula), and it may just increase the panic.
Remember to breathe. Get lots of oxygen in. It's said to improve the ability to concentrate.
Before the exam, make very, very sure that there are no crib sheets accidentally left in your pocket. Even worse than spending 20 minutes on something worthless would be getting caught with a crib sheet when you didn't even intend to cheat and just left it there by accident. In the UK, they can cancel your entire GCSE results on that, and that's your life (at least the version of it you currently cherish) down the drain.
Another benefit of having at least a quick scan through the questions on the paper before you start is that your subconscious gets a chance to mull things over a bit before you actually start the question.
Anybody taking an exam should get to know the exam, as much as possible, way before they actually sit down at that tiny desk in that oppressively airless room. This is what past papers are for... and teachers, hopefully. Then you know what to expect; and you can find out where your strengths and weaknesses are, such as what type of tasks you zoom through without any trouble and which are more problematic for you. It helps with working out timing and strategies, too.
Learning to plan effectively in writing papers is almost as important as learning to write good essays, as is sticking to the word limit, if there is one, as this is also part of the task (but for goodness sake, don't actually count every single word).
Answer the question that you are being asked. Writing all sorts of clever nonsense isn't going to do you the least bit of good if it is about something else.
If you are taking a foreign language exam, where the grammar part of the paper has a text with spaces you need to fill in, read the text before you start filling stuff in.
With listening papers, don't sit there staring into space while the moderators faff around cueing up the tape. Read the questions. Think about the questions. Try to think about the information you are going to hear. Anticipate.
Try not to arrive late. Know in advance precisely when and where the exam is being held.
Go to the toilet before you start. This may not apply so much to boys, or young women; but it can be very difficult to 'hold it', if you're a woman who's had kids. Going during the exam may be OK; but it does distract you and reduces the amount of available writing time. Consuming a lot of caffeine to keep awake before and during the exam will make toilet planning all the more important.