I get a sore back whilst riding my mountain bike ,and on longer trips on my Fazers.
I find yoga excersises very good, especially a series of excercises called "sun postures"
last year my back had flared up to the point where I was taking pain killlers before every ride. but just keeping at the mountain bike and yoga ,the pain in my back has settled down to be almost forgettable. If the pain is arthritic in origin , I think you will find it varies on a month to month basis