With your Right thumb, close off your right nostril and draw a breath through you left nostril. Then, when you're ready to exhale, remove your thumb from your right nostril and close off your left nostril with your ring finger and exhale. Do the complete circuit (Right/Left; Left/Right) 4-5x's.
If you're really tense or agitated, it may take up to 10 minutes before you feel relaxed, but it's a convenient little technique to ward off high anxiety. As with all of these tips to improve mental health and brain function, they have been previously published and referenced. Caution: Do not force, especially if you have any nasal blockages, and, if you have high blood pressure or any medical condition, consult your physician first before trying.