Happens to everybody usually at least once. You can practice getting yourself worked up, with all your gear on, and then just take 1 arrow shot at the target, really pretend it is a deer, and make it count!

Don't worry it gets easier, get a few deer under your belt you'll calm down a little bit, but you'll always get excited, I think we all get used to being excited.

Also you can concentrate on picking the exact hair that you will shoot at, really focus in, focus on your mechanics. (ie squeeze trigger, don't snap, don't move fast, etc) While you are sitting there waiting for a deer go through every angle that a deer will come in from, and plan out where you must have the bow drawn back, and where you intend to shoot. The idea is to be prepared, so that when a deer comes in, you are thinking ok, I will draw back at that tree, plan on shooting at the opening 10 ft in front, etc to get your mind on business, and off of "holymoly a deer is here!"

Now get back out there!