The accepted technique is to breath deeply so that the abdomen actually rises (like an infant breathes) and do it to a count i.e. in for four seconds, hold for four seconds, breathe out for four, hold out for four.
What is happening is that your brain perceives danger and so releases adrenalin which causes (amongst other things) elevated heart rate and blood pressure and shallow breathing. When the brain then picks up on these "symptoms" it says "wow, something is up, more adrenalin" and the release of more augments the aformentioned symptoms and you get caught in a viscious upward spiral, that, if not experienced often, or allowed to get out of control will lead to shock.
By deep breathing in the recommended 4 in, hold 4, 4 out, hold out 4, you trick the brain which registers the slow rhythmic breathing and says "obviously all is calm" and tends to slow things down again.
Nick
PS: (as explained to me by one of the best shots I've ever had the pleasure of knowing who is on our local SWAT team)
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Hard pressed on my right. My center is yielding. Impossible to maneuver. Situation excellent. I am attacking.
--Ferdinand Foch-- at the Battle of the Marne
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