Switch from mouth breathing to nose breathing

Breathe through your nose throughout the day and night.
Whenever you notice yourself breathing through the mouth, close your mouth and switch to nasal breathing.

Aim for nasal breathing that is soft and calm.

If your nose gets blocked, decongest it.
Exhale, pinch your nose and walk as many paces as you can with your breath held. Aim for a moderate to strong air shortage. Repeat each minute for a total of 5-6 breath-holds, or until your nose is clear.

Avoid regularly sighing and yawning.
When you feel a sigh or yawn coming on, swallow it. If you miss one, gently exhale and hold your breath for 5-10 seconds afterward to compensate.

Tape your mouth closed during the night to promote nasal breathing.
To help build up comfort with this, try it out for a few hours during the day first.