Sorry, gonna have to disagree with everyone. Once in a while we get to the point where everything hurts, or is sore, or chaffed, or whatever. Yeah, there goes the "no lube" post I made earlier. Maybe that's why we're sore. So we just lay off for a day or two and we're good.
You need some good furniture if your back is hurting you. I find that a couch is pretty good height for me to do it from behind. The bed is just too high, so we're moving the ottoman into different rooms for sex usage.
Note** If anyone ever visits my house, don't sit on the ottoman.