The most effective thing I have found for relaxing back muscles is somatics:

Hanna's landmark book:
http://www.amazon.com/Somatics-Reawa...dp/0738209570/

A book with better photographs and instructions:
http://www.amazon.com/Move-Without-P...dp/1402774591/

A DVD with the strange title of "Yummy Yoga" since there is no yoga an the DVD. It's all somatics:
http://www.amazon.com/Yummy-Yoga-Gen...dp/B00075K846/

It's neat stuff. Fun to learn.

Also, this small, sensible stretch set really opened up my back with no ill side effects:
http://www.amazon.com/Minutes-Pain-F...dp/0743476476/

The reviews for all of the above are informative.

HTH