I also find that lots of stretching helps. My back has a low level of pain all the time and a medium level of pain during longer or harder bike rides. I find that getting off the bike and doing some gentle stretches while I'm really warmed up works really well in keeping it from going into a full-blown spasm.

I do go to the chiropractor about once a month and get everything snapped back into place, but it doesn't last for long and I'm trying to wean myself off of going there.

I also recently started Feldenkrais lessons (http://www.feldenkrais.com/). It's a method of moving that retrains your body so that you move in a way that is not painful. It's kinda hard to explain, but I am trying it as a natural way to solve my back pain issues.

Most of all, I think it's really important to keep moving.