Just as all belief structures hold to a Golden Rule, so too do all calming and comforting techniques hold to a Golden Rule:
Slow, deep, controlled breaths have the natural effect of slowing your heart rate, bringing your blood pressure down and easing nervous tension. It is this physical act that soothes smokers, brings practitioners of meditation to their Zen state and helps anger management members calm themselves by counting backwards from ten.
I always recommend a 5-5-5 breathing exercise:
Sit somewhere comfortable and quiet, close your eyes and bring to mind a blank, white background. All of your thoughts appear on this background as they pop up. Inhale slowly for five seconds to the fullest expansion of your lungs. Mentally imagine yourself "wiping" the image in your mind clean of your thoughts. Imagine a single, slow swipe across this white background, like a whiteboard.
Hold your breath for five seconds, feel and focus on your heartbeat.
Exhale completely, letting out your breath slowly for five seconds.
Repeat this exercise until you can keep your mental white space "clean", and bring your heartbeat to a slower rate. This is your calm center.
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