My back problems went away when, rather than trying to sit "straight," I made sure I had a gentle curve to my spine in the lower back (curving inward). The "straightness" isn't so much about the spine being straight up and down - but not leaning forward or back. Our spines curve - that's what they do. The stomach is pushed out a little bit. Think about how before we evolved to two legged beings "we" would crawl around on all fours and gravity would pull our middle part downward. (I don't know if that's an anthropological fact, but it makes sense to me.)
I also occasional do this yogic back strengthening/stretching series: cobra, quarter locust (each leg), half locus (be very careful - take a yoga class or read up on how to do this properly before trying it, and be gentle), full locust. After doing this for awhile and sitting regularly I feel like I have a back of steel or something. When I don't do it I definately feel the difference.