Check your bike 1-2 days beforehand to make sure the tyres are legal and it's roadworthy. This gives you time to fix anything that's broken. Do your POWDERS and make sure your bike is cleaned and prepped to set a good first impression.
Get your documentation together the day before, so you're not scrabbling around the morning of the test.
It's test day, you're nervous and you've not had much sleep. Try to wake up 30 minutes earlier than normal, grab a coffee and make sure you're alert before you get on your bike. Don't risk having an accident if you're sleepy.
Meet your favourite Observer for a pre-test warm up ride to calm the nerves. He might know the examiner and can practice some routes he might take from the meeting point.
Fill up before you meet the examiner.
Get to the meeting point early, grab another coffee and relax. Being early shows planning and preparation and makes a good initial impression. That's 1 easy brownie point in the bank.
Use the bathroom before you start the test.