Lots of good advice already provided.Controlled breathing keeps up the oxygen supply and creates a focus other than the animal,I like what Kodiak Archer said,basically have your focus be on your form and executing a good shot at specific spot on the target/animal.I have done a couple of other things as well,self talk,such as telling myself I probably wont get a shot when an animal is coming in that I want to shoot,it is a little bit of denial that is taking the heat off until you know for sure whether you will have a shot opportunity.
There is no substitute for experience and being around game,there is one thing that I have found immensely helpful,not caring so much whether I take an animal or not.I give my hunting everything I can from a time and effort perspective and whatever happens,happens.Some of the best things that have ever happened to me in my life have happened once I let go of being so concerned about the result and just focused on the preparation and enjoying the entire experience.I hope this makes sense to anyone reading this.