I can totally sympathise! I've had a bad back now for many many years and am a poor sleeper to boot, so the PG has knocked me around quite a bit. I must say I'm not as bad as I thought I may be, although my physiotherapist did say to me that it was a possibility that my back would be WORSE after the birth than during the PG! NOOOO!!

Anyway, I certainly have times where I can barely walk or move and am in agony. Heat is a great thing, so yeah, a nice warm bath or a shower are great. Have you considered seeing a physiotherapist? A lot of women will see one during PG for back pain - they may be able to give you some specific to your situation that might help as everybody is different. I also try to swim regularly as the feeling of weightlessness is really lovely, but it's also a great exercise for a bad back anyway, but particularly good during PG.