Back Pain Resource Center - heat and cold therapies to help reduce back pain
By far the best moist heat delivery system I have tried. They are the same ones used by physical therapists. You have to cook them in hot water, wrap them in towels, and place them on your back. As it cools down, you take off towels to increase the heat. Many a day this was very sweet relief for me. Look for these in medical supply stores - go for the large ones that will cover all your back and make sure you get a cheap large pot to cook it in.
I found one of these gel packs in a medical supply store a few years ago. It was big enough to cover my lower back and had a velcro strap attached so that it could be secured around my waist. These packs can be used for both hot or cold treatment - put it in the microwave for a hot pack, or in the freezer to use it as a cold pack. I did find that I had to be careful when using it from the microwave since it could be a bit too hot. Usually, though, I used it as a cold pack and could strap it onto my back and wear it for a while around the house. I found that using it cold helped me with inflammation, so I would strap it on after exercising or doing Pilates. And since it was a gel pack, I could then just stick it right back in the freezer to be reused again and again. I believe the brand I used was Spenco, but in looking at their web site, they don't currently offer this pack as part of their product line. Ask your local medical supply store, and they can probably tell you which of these gel packs is the best bet for using on your back.