Are there any Yoga exercises to help aching backs?

A regular yoga practice will strengthen your back and slowly you will start getting rid of that pain. I would recommend you to attend yoga classes where a teacher can guide you in your practice.

http://s3cr3ty0stg2.wpengine.com/Yoga-for-beginners/How-do-I-start.html

Also, these exercises done with moderation can help you.

http://s3cr3ty0stg2.wpengine.com/newsletter/12-2008/exercises_maintain_flexible_spine.html

Blessings