Looking after yourself - for example, getting enough rest and eating reasonably - is more important and effective than trying to do some last minute cramming. This is a day to have planned beforehand and to take things gently in order to conserve your energy for your exam(s).
Don't get up very early, as this will just make you more tired. Eat breakfast, but do not drink too much liquid! If you have spare time, do something you find relaxing - have a bath, go for a stroll - and keep away from those whose stress levels are contagious.
Rather than trying to learn any new material, perhaps just look over a few key points.
Arrive at the exam hall comfortably in time but not too early; the tension hanging over this short period of waiting just before the exam is highly contagious so you do well to minimise your exposure to it.
It is natural to feel some anxiety when you go into the examination room. Use the few minutes before you are allowed to begin to do some simple relaxation and breathing exercises; sit back and separate yourself mentally from those who are getting stressed.
Read the exam paper through slowly. When you have chosen your questions read them through twice to make sure you have understood and not misread the question. If you are allowed to do so, underline key words or phrases in the questions.
Answer the correct number of questions and divide your time equally between them - or according to the marking scheme if questions have different weighting. With essay questions, you will get more marks overall by doing three average answers, than by doing two brilliant ones but leaving the third question undone.
Some people write out essay plans to all the questions they need to answer at the beginning, so they can add things as they occur to them while working on other answers; others take each question in order. Which method works best for you, or is most appropriate to the format of your exams?
After doing your plan, look back at the question and check you are answering the question asked - you do not get credit for a brilliant answer to a question you were not asked!
Take regular 'micro-breaks': whenever you pause at the end of writing a paragraph or stop to think for a moment, put your pen down and sit back, even if just for a moment.