Before the Test
What helped me was remembering the fact that the examiner is just another person and that I am more current on the aircraft than he/she is, therefore I could probably fly to tighter tolerances than they could.
Try not to give them a mystique or an aura, because at the end of the day, they are just ordinary people the same as everybody else.
Try to focus solely on the flying elements of the test (ie the elements that you can control) try not to focus on the consequences of failing the test (the extra money required, the desire to get first time passes etc) because these will start to creep into your mind as soon as you make a slight mistake during the flight and will start to distract you from the actual task of flying.
The night before the test, maybe go to the gym or swimming pool and do some exercise as this will help you get a good night's sleep.
During the test
It sounds obvious, but the test is not over until you have shut down the aircraft. This means that everything is still to play for right up until the end of the flight. If you have a weak section initially (maybe a poor diversion in your CPL or a ropey first approach in your IR) it can be balanced somewhat by a good performance elsewhere in the flight, so dont get disheartened by thinking you've failed already. The only person that can fail you is the examiner, dont fail yourself (if you get my drift)
From personal experience I've been convinced that I've failed or partialled exams during the flight, but, in reality, the situation was not as bad as I had thought.
If you do make mistakes, just stay calm. Think of it as an opportunity to demonstrate your airmanship and ability by logically sorting out the mistake. Whilst making a mistake is not the end of the world, the examiner will probably take a dim view of things if you sit on the mistake and dont correct it sensibly.
Hope all this helps,
The CAA examiners I've met have all been really decent friendly people who want you to do the best that you can (they dont try to wind you up and get you nervous and agitated before the flight).
All the best