How to help with chronic lumbar back pain without any prior injury?

Back exercises. Start with a back exercise program called "williams flexion exercises". A physical therapist can help you with this and can also apply different modalities of therapy. An evaluation by an orthopaedist will help determine the cause of your back pain.
Conservative care. There are many studies on the subject of lower back pain. The majority suggest the best method of controlling chronic lower back pain is exercise to create core strength and loose weight. I also suggest daily stretching and core strength exercises. You can find these exercises at: http://harvardneurosurgeon.Com/pt2.Html.