The technique of breathing is the basis of playing the clarinet and each clarinetist has to practice it. It is firstly done without an instrument. Very important thing is to learn how to save air consumption and a long breath.
When doing this, we should feel that the inhaled air automatically “drops” into the lower parts of the lungs. Diaphragm (a muscle that divides the chest cavity from the abdomen) curves down and leans on the abdominal cavity. The stomach is arched forward, and the lower ribs are spread in all directions (see picture left). Then we get a sense of simultaneous filling of the stomach downward, forward, backward and from the sides.
Taken from my book