Chronic lower back pain can have a variety of causes. These can be psychological or physical issues. As Bipolars can be prone to experiencing greater amounts of stress and stress is sometimes a cause for back pain, effective coping techniques is a good first step. If you already are dealing with your stress to a reasonable extent, you're good to go (psychologically).
If there is a physical issue involved as well, you have three major choices:
a) seeing a back doctor (most severe -- if you will require surgery)
b) seeing a chiropractor (controversial -- works for some, but did not really work for me)
c) going through physical therapy
You can do physical therapy at home. Almost any ab excercises are good. Best practice for crunches is for your elbows to touch your knees on each rep. The single most effective excercise is to gain endurance on airchairs, against the wall. Make sure when you do airchairs that your lower back is touching the wall (your whole back should be flat against the wall). Time how long you can do the airchair for and build up to a couple of minutes. As the minutes build up, you should start to feel the pain going away on a regular basis.
Swimming helps as well.