I presented this same question to a co-worker of mine and he gave some pretty good advice. I know this sounds stupid but it really helped me last year. What he said to do was when you see the animal coming and the anticipation is starting to build, calm down and count down from some number by an irregular count. Like count down from 50 by 7's. It makes you think taking of something else which helps to calm your nerves. Like I said,I know it sounds stupid but it really didhelped me to calm down and focus on the shot.