Whenever you experience mental tension, try breathing slowly and rhythmically for a few minutes while you concentrate on the breath. Yogic breathing exercises will may take conscious effort, but develop your ability to calm the mind using your own thought power. This leads to an experience of inner peace,with physical relaxation following the mental relaxation.
From: Brooklyn, NY
Referenced from: http://yoga.org.nz/what-is-yoga/yoga_relaxation.htm