Make sure you drive around with your instructor for an hour or so before the test to 'warm up'. It's quite helpful, especially if you're testing in their car.
And, this may sound pessimistic -but it's really not meant to be.
Don't expect to pass on the first try.
The reason I say this is that if you don't expect to pass on the first try, perhaps you won't put so much pressure on yourself, and it may help a little with the nerves.
Many people (inluding myself) don't pass on the first try, so if you happened to fail the first time, you would be in quite common company. And for the second test, at least in my case I was much less nervous because I knew exactly what to expect.
And If you do happen to pass the first time, excellent, it'll be a pleasant surprise!