Rest was standard of care 30-40 years ago. We stopped doing that because inactivity leads to muscle atrophy and further spinal stabilizer muscle atrophy, thus greater spinal instability.
It sounds weird, but the best I've found is running. When I'm in running season, I tend not to get back spasms, and if I do they are mild and only last a day or two. In the winter when I don't run, a back spasm hurts like hell for a week or longer. I'm guessing it's my shoulders moving up and down with each stride? I don't know, but something about it helps immensely.