Hey, I think we all get it. What I do is I try to talk to myself, step by step of what to do while trying to control my breathing like Wiredtohunt said. You can also practice, by running around the house or going for a nice jog, then shoot your bow or gun. This will be about the same as it would be when that deer steps out.
"If I pull the hammer and shoot this young buck, he's dead. But if I pass on him, the next hunter might not shoot so straight."