Exercise ball worked well for me.

I tried a halfway solution, one of those round inserts you can put on an office chair, and it actually made my posture much worse. Standing desk would be tough for me where I work, and I tend to stick out my hip and lean to one side when standing for long periods.

But the ball was great. I think the primary mechanism for posture is that it makes me notice immediately when I'm slouching, since it's uncomfortable when I do so (while on a normal soft chair slouching is relaxing).

The first day on the ball I could only handle a couple of hours, and my legs were sore for a few days afterwards. But after day 3 it was no more difficult than sitting in a chair, and less difficult than sitting up in a chair. If you try it and after a week or so you don't feel good, then it's probably not for you.