FFS....
Be early
ensure all your paperwork is up to date
neat and tidy uniform, including polished shoes/boots
fresh breath (ie no tuna and garlic sandwich for lunch)
SHAVE
haircut
all docs amended
ensure aircraft has NO defects, and plenty of hours to run
basically.... you WILL know what to do, else your instructor will NOT put you up for it. NERVES are the killer! trust in your training, and don't bullsh!t. IF you don't know something... say so, and if you know where to find it... say that also!
the examiner KNOWS that you will be nervous, but he/she has a SET schedule of items that they need to test you on. you should KNOW in advance what these things are.
ie: Q&A prior to flight, flight plan, and reading wx, normal take off, x country exercise with diversion, instrument flying, circuits, controlled airspace, engine failure(s), prec search....