Strengthening the muscles around lungs helps deep breathing. The muscles related to breathing is thin and usually not used so they are strengthened effectively by simple stretches.
(1) Breathe in from your nose for 3 seconds, holding your hands behind your head. (Fig. 2)
(2) Stretch your hands above your head, breathing out from your mouth in 6 seconds. (Fig. 3)
It is recommended to do this stretching 10 times in 2 sets every day. At the earliest, the effect appears in a week.
A 50 years old male tried this stretching for 2 weeks. The age of his lung function was 57 years old before starting this stretching and 2 weeks later it became 45 years old.