You need a good chair.
It was just impossible for me to sit up straight in my old chair, which had no back support at all. I knew it had to be replaced when I found myself adjusting the center post every 20 minutes. I hate most office chairs - they’re not tall enough, and they have the wrong kinds of adjustments. I like the Aerons and Mirras, but they’re pretty expensive, even used.
This chair is a pretty good compromise - not too expensive, tall (it’s the only chair I could find that I could sit in comfortable at less than the maximum height), and with a good lumbar support.
Plus, it’s on sale this week and staples.com has free delivery.