One of the earliest references to breathing as a form of medical therapy appears in an inscription found on 12 jade tablets dating from the mid-sixth century BC:

In breathing, one should proceed as follows: hold the breath down and let it collect together. When it collects, it will expand. When it expands, it sinks down. When it sinks down, it grows quiet. When it grows quiet, it will solidify. When it solidifies, it begins to grow. As it grows, it is drawn inward and upward and will reach the crown of the head. Above, it presses against the top of the head. Below it presses downward. Whoever follows this method will live a long life. Whoever goes against it, will die prematurely.

I found this useful in combatting the shallow breaths most people (myself included) take as a matter of course.