Take it as read that you will make some mistakes, or not do things quite as perfectly as you would like. The key is, if you think you've stuffed up, put it behind you and don't dwell on it.
I thought I'd potentially failed my PPL at about 3 or 4 different points during the test, but I think what the testing officer wants to see is that you are safe, you are sensible, and you can deal with the situation, and I actually passed. During feedback, the things that I'd thought were end-of-the-world foul-ups, the examiner just passed off as the sort of things that you'll fix up with experience. The key was that I was safe and in control at all times.
I think the other good piece of advice is start making a good impression from the outset, and the examiner will begin to form an impression of you as a competent pilot before you even get near the aeroplane. Have all your paperwork ready and organised, if possible have a desk set up and ready to go for all the preamble, dress smartly - nothing over the top, but trousers, shoes and a collar of some sort make you look like you are taking it seriously. Get there early, make sure you're fuelled up, oil level okay, anything you can do to make life easier for yourself.
And to echo what many have said - you begin the test with a pass, it's up to you to fail it.
Good luck to all who have them coming up, it's a great feeling to finally become a pilot!