1. Use your nose
Breathing through the nose can slow the breath and make the lungs work more efficiently. It also facilitates the intake of nitric oxide, which helps with oxygen transportation throughout the body.
It helps: 

However, breathing through the mouth is sometimes necessary if a person is exercising or has sinus congestion.

2. And Your Belly
Humans are “belly breathers,” and just above your stomach is a major muscle in the respiration process, the diaphragm. Proper breathing starts in the nose and then moves to the stomach as your diaphragm contracts, the belly expands and your lungs fill with air. “It is the most efficient way to breathe, as it pulls down on the lungs, creating negative pressure in the chest, resulting in air flowing into your lungs.”

(via: The American Lung Association)

3. Breathe with the book
A therapist showed me how to relax and strengthen the diaphragm: Lie on your back, a thick book on your upper abdomen. Now breathe in on a count of 5 while lifting the book. Exhale on 5 and lower the book again.

(also helped me with tachycardia)