It really helps to know the area the test centre is in a bit - you can't drive all the test routes, of course, but just being generally aware of the types of roads around the centre will mean that you're less likely to be phased on the day.
Make sure that your mirrors, head restraint etc are well positioned before the test, so that there's no chance of forgetting to adjust, etc,
Don't sit in silence in the waiting room, if possible... I found chatting to other people and my instructor really helped with my nerves.
If your examiner feels like you can manage it, they might ask you what you'd be doing if you weren't on the test or something similar. If you know that you can't multitask or that you're easily distracted, just ask them politely as you usually would if you could drive quietly - they won't mind but it can make a difference to you. For me, it was a benefit to talk to the examiner, but it really depends on the person!