NAro's advice on breathing is exactly right. 12-year SEAL veteran & trainer
Jeff Gonzalez reports that this practice helped him enormously during his military operations. Trainer
John Farnam says the same thing. I believe Farnam refers to the practice as "autogenic breathing".
From
http://www.killology.com/art_onkilling_anxiety.htm:"...the bridge between the somatic and autonomic nervous system is breathing, and an increasing body of research and law enforcement experience indicates that if we teach the victim to control their breathing then they can control their physiological arousal. (This is based on information and feedback gained from training over 20,000 law enforcement personnel in this technique over the last three years.) The breathing technique that is being taught to SWAT teams, police departments, Green Beret battalions, and other elite forces around the world (sometimes referred to as “autogenic breathing”) consists simply of a deep, belly breath: breath in for a four-count, hold for a four-count, breath out for a four-count, hold for a four-count, and repeat three times."