Relax
In order to get the most out of each breath, your body should be as relaxed as possible. Consciously try to relax your shoulders, your chest, your neck and your head muscles. If you don't know how to consciously relax a muscle, you can clench it for a moment then let it go completely.
Visualize
With every breath you take, visualize the oxygen from the air spreading out from your lungs into every part of your body. This kind of visualization encourages the development of the right kind of awareness of your body for yoga.
Don't Fight the Breath
Although you are supposed to aim for a certain kind of breathing in yoga, it is even more important that your breathing is unforced and natural. If your breath wants to change, don't try to force it into the pattern you want, just let it change. Then try to guide it slowly and gently back into the desired pattern.
You will find that your yoga sessions go more smoothly and produce better results when you master the art of yoga breathing. It helps to practice the breathing method even outside of the yoga studio, separately from the poses and movements. When you get enough experience, the right kind of breathing will come naturally to you in your yoga practice.