Proper breathing is probably the single most vital element to a regular yoga practice. I not only keeps you grounded and focused, but it moderates your heart rate, drives oxygen to the muscles and ligaments for greater extension, and to the brain to increase it's cellular activity, but it also calms the spirit and energizes all the chakras and meridians of your body. Oxygen is literally your life-force. Without it you cannot survive and increasing your ability to take it in and your control over it will help you in many other areas besides your yoga practice. It's essential that you don't forget to breath during transitions from asana to asana. Try if at all possible to abstain from inhaling through the mouth, and instead take in air through the nose. It's OK to let it out through the mouth however. This takes repetition to master and can be a challenge, especially if you are a shallow breather. Don't be discouraged, as with yoga and with everything else, practice makes imperfect. Happy breathing!