Purse your lips and blow out all the old air in your lungs (direct it downward, away from others) engaging your belly muscles to fully empty the lungs, immediately pause at the bottom of your exhale.
WAIT for a feeling of slight discomfort as you pause after the exhale, before you decide to inhale through the nose, enjoy the fresh newly oxygenated air coming in, deep. Notice how deeply the lungs fill now that the old air is cleared out! Sigh contentedly. Allow the mouth to relax toward a smile.
Aim for full and regular-spaced breaths, adjusting the breath any time you notice you are breathing shallow or delaying your breath due to heavy concentration.
Try a deep breath or two any time you notice you have been breathing shallow or delaying your breath as you going about your day.