If there's just one thing I could say to those with back pain, it's this: ride your bike, a lot. After I injured my back skiing after years of not working my back, cycling was what made the pain go away. That's when I took up randonneuring for a few years.
Of course I also worked my back, hard, stretched and etc.
If one is just starting out to rehab a bad back,
Core Advantage with Tom Danielson is a good start. After my ski injury, I spent a year slowly progressing through the exercises in that book. Took ~20 minutes every morning. And ride your bike. I've always used a full-on road position, bars as low as my frame allows and with a good long reach.