The most important and simple thing to do is to re-establish the natural breathing pattern. This can be done by breathing out fully first before breathing in. When you breath out fully you become relatively oxygen starved which will then be the natural trigger for you to inspire.
As inspiration occurs follow the expansion of the abdomen outwards (not upwards). If this pattern of breathing is learnt it activates the natural subconscious pattern of breathing that a baby is born with.
This method breathing training was developed by the Russian Dr Buteygo and is now a standard means of respiratory training used by Respiratory Specialists. The Buteygo method is based on raising the body’s carbon dioxide levels which is the trigger in the brain for inspiration to raise the oxygen level and lower the carbon dioxide level. For example, the classical treatment of somebody hyperventilating is to raise the carbon dioxide level to sedate respiration (nervous system) by obstructing the nose/mouth so they can rebreathe old high carbon dioxide air..
By focusing on expiration this allows inspiration to naturally occur. The natural breathing will then become re-established and become the subconscious way.