I first learned about pursed lip breathing when I went to pulmonary rehabilitation. This way of breathing reduces the number of breaths you take by slowing your breathing. This keeps your airways open longer, allowing more air to flow in and out of your lungs. It also strengthens your respiratory muscles. Simply breathe in through your nose with your mouth closed and breathe out at least twice as long through tightly-pressed (pursed) lips. I breathe in for a count of two, and out for a count of four. You can use pursed lip breathing when you are short of breath to get your breathing back under control and calm yourself down.
I recently finished a book by Richard P. Brown, M.D., and Patricia L. Gerbarg, M.D., called “The Healing Power of the Breath: Simple Techniques to Reduce Stress and Anxiety, Enhance Concentration, and Balance Your Emotions.” I found it to be very helpful.