The best way I've found, short of all the excellent choices above is to simply breathe. I've been going to a sports psychologist ans he'll have be breathe at 6-7 second counts. The only difference is on the exhale, fully expel all air from your lungs. Just close your eyes and actually count off the seconds in your head.He's actually put me on a machine to measure and after 1-2 minutes my heart rate will drop 10 beats and blood pressure 10-15 on top and 5-10 on bottom numbers. While shooting I try to focus on my breathing and really focus in my between shot routine. Hope this helps!!