"You cannot do it," explained the Master, "because you do not breathe right. Press your breath down gently after breathing in, so that the abdominal wall is tightly stretched, and hold it there for a while. Then breathe out as slowly and evenly as possible, and, after a short pause, draw a quick breath of air again -- out and in continually, in a rhythm that will gradually settle itself. If it is done properly, you will feel the shooting becoming easier every day. For through this breathing you will not only discover the source of all spiritual strength but will also cause this source to flow more abundantly, and to pour more easily through your limbs the move relaxed you are."

...

The Master attached so much importance to breathing out as slowly and steadily as possible to the very end, that, for better practice and control, he made us combine it with a humming note. Only when the note had died away with the last expiring breath were we allowed to draw air again. The breathing in, the Master had once said, binds and combines; by holding your breath you make everything go right; and the breathing out loosens and completes by overcoming all limitations.

...

When, to excuse myself, I once remarked that I was conscientiously making an effort to keep relaxed, he replied: "That's just the trouble, you make an effort to think about it. Concentrate entirely on your breathing, as if you had nothing else to do!"

From "Zen in the Art of Archery"
by Eugen Herrigel
whose Master is Awa Kenzô (1880-1939)