Happily, I discovered a book on back pain [ http://www.amazon.com/Steps-Pain-Free-Back-Solutions-Shoulder/dp/0979303605/ ], followed its instructions, fixed my posture, and the pain went away almost immediately.
That was two years ago, and my back hasn't been giving me any trouble until this weekend. I started getting aches and twinges, and I'm hoping the problem isn't coming back. But my guess is that I've just been getting lazy and sloppy with my posture, and my body is reminding me to get with the program.
Just to be on the safe side, though, I went to the thrift store and bought some halfway decent furniture - a couch and two chairs. For the past year I've been making do with a two-seater couch that's too low and too soft, and kitchen chairs and folding chairs. That shit is getting old, and so am I.
So, now I just have to arrange to get the furniture delivered. And remember to practice my good posture.