Breathing during Asanas

By learning to coordinate your breath with your movements, you'll find that you feel much more at ease in the poses. The rules governing whether your inhale or exhale- and when-are dictated by the direction in which the spine or limbs moved. ...

On the Inhalation:
Inhalations are generally mated with upward or expanding movements. Going into a backbend such as the Cobra, for example, you begin on an in-breath. Then you simply hold the pose and breathe rhythmically. An exception to this rule: Upward movements of the legs work best on the exhalation since the legs are much heavier than the arms.

On the Exhalation:
Exhalations are usually mated with downward and contracting movements, such as lowering the arms, and with any positions that employ flexion of the spine (i.e. folding the body into itself such as in forward bends, abdominal curls, lateral stretches, or twists). When you lift a substantial weight, exhale on the effort. This applies whether you're lifting a 10-lb dumbbell or your leg. The out-breath helps contract the abdominals which in turn stabilize and protect the lower back.

From Yoga Journal's Yoga Basics by Mara Carrico