Its like a good golf shot. Just hast to happen. Forget all the bad shots and touch that trigger (hit the ball) like you've never missed a bullseye (or fairway) in your life.

In both golf and archery one tip that really works for me is to make the bullseye (or hole) a lot smaller than it actually is. Pick a spot on the bullseye (or a grain of grass on the back of the cup) and hit it. Your bad shots will get a lot a better when your shooting for something a lot smaller!

But to answer your question, exhale softly. Thats what snipers do.