I was told to do this by a doctor a while ago and never actually tried it until now because I was wearing a splint and it was too hard to sleep with to begin with let alone try to get used to an awkward sleeping position. However now that I am not wearing the splint I actually tried it and I think it is something worthwhile:
Take a normal pillow and roll it up, keeping it the same length, so it is basically a cylinder. Use some duct tape or other strong tape and tape it together securely. Put it back in a pillow case.
Now take any other pillows off your bed so you can't end up using them while you are sleeping, lay on your back so your neck is resting on the new pillow you made. This is supposed to keep the head in a position where gravity makes it hard for you to grind your teeth and basically forces you to relax your jaw. I tried it last night and plan to force myself to sleep this way from now on as I am already feeling somewhat better. I doubt it can work for everyone but at least if your TMJ isn't too far along it should help.
I'd like to hear if anyone else has tried this before and what they think of it.