Look son, being a good shot, being quick with a pistol, that don’t do no harm, but it don’t mean much next to being cool-headed. A man who will keep his head and not get rattled under fire, like as not, he’ll kill ya. It ain’t so easy to shoot a man anyhow, especially if the son-of-a-bitch is shootin’ back at you.  — Unforgiven

I read on one of the EMS blogs once that a paramedic does not run, he mosies.  It is amazing how more effective someone is when they take the time to think before acting, even in an emergency.  Also, when you seem to be cool and calm, that feeling will spread to others.  Even if you get rattled, a moment to find that inner silence will steady your hand, slow your thoughts, and improve your chances.