Always make sure you know exactly when and where your exam will be. Every year some students get the date or time wrong or go to the wrong room. Don't be one of them!
Check you have the right equipment for the exam. If you are allowed to use a calculator, take it even if you don't think you'll need it. Take at least two or three pens with you in case you run out of ink or one of them stops working.
If you want to, take a 'lucky' mascot in with you. It won't itself bring you luck, but it can be comforting to have something familiar on your desk to help you stay calm.
Find out in advance which tier paper you will be taking (higher, intermediate or foundation) and always
check in the exam that you've been given the right paper. If you haven't, ask one of the invigilators for the correct paper before the exam begins.
Get comfortable! If you've got a wobbly desk or chair, ask for a bit of paper you can fold up and put under one of the legs.
Make sure you take time to read the instructions on the front of the exam paper. If the paper says to answer three out of the four questions, then only answer three questions! It sounds too silly to mention, but there are always people who answer the wrong number of questions or answer questions from the wrong sections.
Don't rush straight in and start scribbling down everything you know. Take about five minutes to read through the questions and think about how the examiner wants you to answer.
Look at the number of marks given to each question. Don't spend half your time answering a ten-mark question if the rest of the exam is worth forty marks.
After the exam, don't worry about whether you did well or not. The worst thing you can do is get into a state about an exam you've just done. Try and forget about it so you can concentrate on your next one.
Finally, remember that exams are not the most important thing in life. Do the best that you can and no-one can ask anything more of you. If the worst happens and you fail an exam, it is usually possible to do re-sits, but at the moment just concentrate on revising hard and doing your best. Good luck!