I've suffered back pain for years and have tried a number of solutions, a few chiropractors i found where rubbisn, but my current one has been brilliant.
I would say main thing is to make sure that the bike is set up correctly and that your back is flat when you ride and bars are high enough that you are not placing to much pressure on hands/ wrists.
The main thing though if you are having problems is too build up core strength, i got my chiropractor and a physio before them to show me some simple lower back stretches and strength building exercises (and a Pilates for Cyclists DVD that i do in private!) and I must admit that I have gone from regular treatment and pain on most rides, to going months without back pain.
If you are stuck at a desk for a long time, or have a job that takes it's toll on your back, make sure your posture is good, desk it set up right etc, no good starting a ride with a spine that is mullered from hours at desk, and where the core muscles are already tense.