Get a good night's sleep before the examination:
Maybe it would be better to say "Get the best sleep you can the night before the examination!" You will do better when you are rested.

When you are preparing for the examination, leave the night before the examination as free for rest and relaxation as you can. This is not the night to stay up late studying or practising!

Eat a light meal:
Hunger (or heartburn from overeating) can take your mind off the examination.

Warm-up lightly before the examination:
during your normal lessons you are taught the importance of adequate warming up for your muscles. One way to get the blood pumping is to take a brisk walk immediately prior to the examination.

This can be particularly important if you are taking the examination in the early afternoon.

Find a way to focus on the examination immediately before you begin:
Everyone has their own way of clearing their minds of unnecessary thoughts.

Some people use breathing exercises, others develop a mental image of a place they find relaxing.

Use whatever technique works for you.

It's important that you be able to concentrate on the exam at whatever level.

Practice makes perfect:
Adequate practice before taking the test can significantly decrease your anxiety.

A timetabled plan will help you reinforce good practice habits so that your skills (and your scores) improve with each examination.