The Day of the Exam

It is important to be prep eared mentally on the day of an exam, apart from having studied hard and done lots of past papers. It is important never to feel rushed or tired. Make sure you are well slept and rested. I used to get to an exam room several hours early and spend the time before the exam thinking into empty space. Arriving several hours early also means you can check you have arrived at the same place to take the exam - useful if an institution is spread over several campuses, or you are an Open University student.
Don't take the car or bus or train to the exam. Cycle. The exercise makes your brain alert and ready for action. Actually, get used to cycling everywhere full stop. You will concentrate much better.
DOn't vary your routine around the time of an exam. It can break your concentration.
Make sure your mobile phone is fully charged and that you have any phone numbers you need, like the number of the exam venue.
If your exam involves writing essays, a lot of the questions are predictable, so have well written specimen essays prepared. Do not under any circumstances leave before the end of an exam. There is always something to check or an answer to be enhanced.
Once out of the exam, don't try and guess your grade by adding up your marks. Grade anxiety may mean you are wildly inaccurate. I marked myself down to a grade C in an exam for which I scored 100%.