I discovered that my SI and back hurt much less when sitting if my legs are straight out in front of me and I'm slightly reclined. At home, I've taken to working, reading, and watching TV sitting propped up in bed. At work, I've adjusted my chair so I lean back and can prop my legs on the back rail of my desk.

I still have to get up and do some walking and stretching every hour, but I am much less sore when sitting in a better position.