In answer to an excellent question in a session I taught last Sunday by a participant, below is a table outlining how to coordinate your breathing while practicing the Intermediate Sun Salutation. Stage I, II and III represent advancement in practice.
As a general guideline, you would inhale as the spine lengthens and exhale when it is released. You can find more about breathing coordination and the impact correct breathing has on the body in an excellent chapter in the book Yoga Self-Taught. You can find a review of this book and many other Yoga related books/music at Ash’s reviews on Amazon.com.
Remind yourself the need to smile when you remind yourself the need to breathe!
[From an Oct 2005 email sent to students]