Playing guitar seated. How is your posture?

I'm not a tall man - (5'7"), and I have been far fatter than I am now, and also (after cancer treatment) far thinner. I participate on some acoustic guitar fora, and often see comments from people complaining about bacl, shoulder and other discomfort - especially when playing larger guitars. When I look back at some of my older videos I see my right shoulder hunched up with my spine bent. Here are some suggestions for minimising issues when playing an acoustic seated.