To quote a famous person, "I feel your pain." Or, at least, I used to. I went through nearly a decade of lower back pain that was caused exclusively by athletic endeavors. My "cure" came about totally by accident. I took a yoga class (way before these things were cool) at my local community college. All my lower body pains left me within a few short months and I really haven't had to deal with them since. The key activity for me was stretching my quads. A simple laid-back hurdlers stretch a few times a day keeps my back happy and pain free.
A few things about stretching: If your back is already in pain, skip the stretching and just rest it. Also, always move slowly into and out of a position, never jerk. I don't think it is so important to stretch immediately before a ride, but always stretch after a ride and try to make it a habit to do it once or twice during the day, even if only for five minutes.
One other helpful exercise other than stretching: Lay on the ground on your back with your knees up and your feet flat on the ground. Now force your back to get flat onto the ground. Hold it there for about one minute (less to start). Repeat two to five times. This odd little exercise even helps when your back is in pain.
Definitely make a friend of aspirin. The pain is usually caused by the inflammation, so get rid of the inflammation. If you can't use aspirin, find a physician who will help you find an anti-inflammatory that you can tolerate.
As far as on the bike, be sure to mix some pancake flat days into your weekly regimen. If you are going to strain your back, it is going to be while climbing a hill. The flat days let you work on form, build pedal speed and strength all without much risk of aggravating your back, as long as you don't smash into any potholes.