If you concentrate hard throughout the flight, you will probably forget about your nerves. Prepare the nav leg thouroughly and make sure all your info is clear on your plog. It will save you from those minor panics in the air when looking for a frequency etc.
If think you've messed something up, don't let it play on your mind. Think about the next thing you've got to do. Chances are that a mistake may not be as serious as you think and it's just possible the examiner hasn't noticed.
If you go out of tolerance on altitude, heading or speed, it doesn't mean an automatic fail. Make a smooth positive correction and you should be alright.
Remember the examiner is on your side and has been through quite a few test rides him/her self. He will understand how you feel. If you mess up something on the gen handling, you might well be given a chance to do it again. It happened to me on my PPL skills test on one of the stalls.
Good luck
