Experience will help quite a bit.As already suggested harvesting as many doe's as the law allows lends itself to taking the butterflys away.For me passing up smaller bucks has also lent itself to me being calmer when I have an opportunity at a bigger buck.
I have found that if I can make sure that I am warm it helps to keep the tension out of my body.If I have a bit of a chill and I get excited I am much more prone to shiver.This also may sound funny but I will use any mental trick I can to occupy my focus,I will keep telling myself it is no big deal it is just a doe with horns.The best thing for me is to completely focus on my form and shot execution,if your form and execution are good,your shot will go where it is supposed to.