Change position. Reading this I noticed I was sitting in the 'good' position, but through the day I also lean a bit forward, all the way, to the side. I'd love an automatic desk that let me stand part of the time.

If you do like to sit straight, look for an older traditional 'secretary' chair with the pad that pushes the mid-back forward to keep the natural curve. Backs are NOT straight, there is a slight but vital curve that many chairs ruin.