Wear comfortable clothes during exams
It sounds so obvious and simple however, many people don't do this. If you wear your school uniform, make sure you have a jersey or a jacket with you as the room may get cold. Remember, you're in an exam for up to three hours and the weather can change quickly.
If your clothing is tight or uncomfortable you can very quickly lose focus. You will be thinking about how uncomfortable you are and you'll lose your concentration. Wear comfortable, loose clothing when you're studying and also during your exams.
A great tip a student once gave me was to use your jacket or jersey to sit on during an exam. It makes those hard chairs a bit more comfortable