Diaphragmatic breathing should help: http://en.wikipedia.org/wiki/Diaphragmatic_breathing

Also, when I had a panic attack in my French prelim, drinking a glass of water helped calm my breathing down. I doubt bringing a glass into your exam would be exactly practical (only reason I got one was because I had to get taken out the room into the languages base ), but you should be allowed a water bottle, at least.

Good luck!