I used to have terrible "programmer" posture. It's still not great, but it's way better. I credit a regular routine of loaded barbell squats with an experienced trainer. I think part of the progress is because I got stronger, but part of the progress is because the squat would point out quirks in my body that simply had to be fixed in order to continue progressing.

There's only so much of "try to sit up straight" before you just go back to sitting the way your body has adapted to sit. In general, your body doesn't want to change. You need to provide some sort of external stress that forces an adaptation. Squats are great for this.