Remain cool, calm, and collected!
Remember, the more panicked you are, the less clearly you will be able to think and recall facts. Before you even think about putting pen to paper, read through the paper in its entirety. Get to grips with what you are being asked and plan out your timings for each question. If you don't know the answer for a question, don't panic! Make an attempt to answer it as best you can, because you would be surprised at how many marks you can actually pick up! If you write nothing, the examiner is forced to give you zero marks! If you're completely stuck, put a star beside it and come back to it at the end. The main thing is not to stress!