A brandy or two helps to calm the nerves...
Rescue remedy might help, its also got booze in it.
Seriously though, don't worry, it's not a competition and anyone who sees it that way is kidding themselves. A lot of it is luck of the draw, down to how it is on the day. Just try your best and you should pass. I failed my first test on a silly nerves based mess up in the reverse park, second time round and I was unexpectedly landed with the relatives old Corsa. Was sure I'd fail again. And then on the day it started pissing down. Was sure I'd fail, yet I passed. Deep breathing and the likes might help. Don't arrive incredibly early and sit around in the waiting room for what feels like an eternity.