I hurt my lower back last year. Thankfully it was a soft tissue thing and not permanent. I had to go to six weeks of physical therapy and while I was there the therapist told me, unsurprisingly, that sitting in front of a computer all day would mean my back problem would keep coming back.
But she had a great idea. She suggested that I write a program that makes me stand up from my chair every 20 minutes for 20 seconds. I've been doing it for about a year now and it has been a great thing for my lower back. I run a Mac (OS X 10.5, Leopard) at work, so here is how I did this.