Sometimes I sit on it, sometimes I lean my back against it, sometimes I do a little bit of both (squish it in half), and sit on the edge of it. I got mine at a sporting goods store.
I am long legged and thought standing works for me and my posture is fine, sitting is a mess and I slouch because my knees are always higher than my hips (I can NEVER get a chair to go high enough). Plus when I am bored I attempt to stand on it and help my balence. I love the one in my car especially because when I am going to a workshop or something I take it with me. It eliminates most of the problems. Your chiropracter might have suggestions for it for you. The nice thing about it is that you can add/subtract air from it as needed. I know lots of people use the balls to sit on but they don't make those tall enough for me either.
Again I don't have chronic pain, but this has helped me to prevent neck/back discomfort. I really just need to figure out how to get myself a treadmill work station.