Before the Test:
It is normally a good idea not to broadcast to too many people that you are taking your test as it puts on added pressure.
Try not to worry the night before. I can appreciate that this may be easier said than done. Make sure you have your driving licence to hand. You really do not want to be hunting for a missing licence before your test. Make sure that it is not left in a family members's car for example, because if they go off with it..... Do everything you can to get a good sleep
If you weren't ready for your test then I would have sorted a test swap for you. I aim to teach my students to a higher standard than the actual test so when the nerves kick in and your driving becomes worse, you still have a suitable standard to pass the test.
Remember that a test is only a test. The examiners do not have to abide by quotas, despite some popular myths. Everybody starts with a clean sheet. What's more important than passing a test is the ability to drive safely. If you are unlucky enough not to pass your test in one respect, in another it gives the opportunity for further learning and to become a better driver.
It's probably best to take the day of the test of from work or other commitments (especially before the test).
Make sure you have all the things you need to bring to the test, double check that you have your driving licence and any glasses or contact lenses if these are needed.