An exercise ball is a good idea but I would not count on using one as a permanent replacement. My ideal situation would be an alternation between an office cha

An exercise ball is a good idea but I would not count on using one as a permanent replacement. My ideal situation would be an alternation between an office chair, like an aeron, and a stable, four-legged cushioned chair (the kind that is usually on the other side of the desk in an office). At home, I switch between an office chair and a piano bench. You don't need a backless chair to sit upright and not slouch. However, an unstable surface (like a chair with rollers, or an exercise ball) can be distracting and requires more energy to maintain an upright posture. So: a bench or a stable chair for posture, and an aeron or similar when your back gets tired. An exercise ball to mix it up would be cool too.