Here is one for you - a nice little free booklet of exercises to assist in correcting anterior pelvic tilt, a common cause of back pain.
I know that stretching out the psoas can often relieve my back pain and this booklet explains why a tight psoas can be a problem, tugging your pelvis out of position.
Mobility work is another route to loosening our muscles of course (as in Scott Sonnon's booklet). Anyway, check out the booklet:
The guy that wrote that has some good free videos on Youtube too, for example