We have nothing to fear but fear itself.
Good preparation from the start is essential.
If the exam is in the morning, all that you need should have been laid out the evening before so as to prevent a last-minute panic. Obviously that means all writing implements, geometrical instruments, calculators and so on; these days it also can mean certain texts or pamphlets that you are permitted-or in the case of 'Open Book' literature exams, require. Most exam centres can lend you a pen or whatever, but they are most unlikely to have a stock of set texts, so be warned!
Get up early enough to leave yourself plenty of time. You may be tempted to get up really early for a last intensive revision session, but I'd advise against it:
The freshness and energy you lose will not be compensated for by the small amount of knowledge you might gain.
Try to get ready in as leisurely fashion as you can manage. If you've got time, listen to some music or do something that you know has a relaxing effect. On the other hand, allow plenty of time for any journey that has to be made-don't let a delayed train or bus ruin things! But don't get to the exam hall too early-about ten minutes before is ideal. Any earlier, and you'll have an empty time in which to panic or expend a lot of nervous energy uselessly; any later, and you'll be rushed and won't have time to compose yourself.
Much of the same advice applies if the exam is in the afternoon. Here, however, a gentle look over a past question or two can be a good way of 'ticking over' in the morning without losing too much energy. If there are some quotations or formulae that you've always found hard to memorize and which you think may be useful, it's worth trying to get them into your short-term memory. But launching into full-scale revision is as unwise now as it would have been in the early morning; besides, if