I've always tried to sit with better posture but I found that if you don't have strength in the right places, a conscious attempt at better posture can sometimes make things worse. In my case, I tried to follow the advice of "sit up straight" by curving my spine back to make up for inwardly rotated shoulders.

The "face pull" exercises in this routine improved my default sitting posture, and I never have to think about it or try to change it. My shoulders and upper back feel a lot better throughout the day after doing these for about two weeks.