At school I was taught the following method for doing exams:
1) Read through all the questions and write down any notes you think may be helpfull on a spare sheet of paper.
This helps clear your mind of all the things you know you will need and helps you to recall other information deeper in your memory.
2) Answer the questions. If you get stuck on any one then skip it and, if you have time, come back to it later.
There may be questions later on which you find easy and you don't want to miss those for the sake of one you find difficult.
3) Spend any time after completing all the questions and before the end of the exam going through the exam again from the start checking your answers.
With reference to point 3, when I did my IMCR exam I found two questions for which I knew the answer but had marked the wrong choice on the answer sheet. I don't know why but I am glad I spent the last 20 mins checking my answers.
Good luck.