Try to switch off an imagine you are ona lesson dont think of it as a test. Thinking to much about what you are going to do can cause you to make more mistakes.
One old timer examiner told me that folk should take an asprin to thin the blood as stress levels get extremely high causing blood pressure to rise. The asprin thins the blood and eases the tension.
he also told me that this could also bee a placebo effect making folk think that the asprin whilst not doing any harm it wont really do any good either.
You should try and eat something as well due to adrenilin dpleting energy reserves if you have some food you wont suffer the same from fatigue as it wears off.
U- Turn dont put your foot down and keep the revs up an old tip dont touch the front brake and remember the examiner cannot see your right foot (back brake) as you make the turn.
Speed limit move quickly up to the limit and stay there.
And finally dont swear at the examiner they tend not to forget things like that and be careful if you joke with them...dont ask how I know this just take my word for it.
best of luck see you on the road soon.