First read the 'sticky' posts above. Some of the stuff is great, like the youtube exercises from a dentist. Don't do anything without going to a dentist/doctor experienced in TMD.
If you already have your appointment, and are so disabled that you aren't sleeping or functioning, you can try mouthbreathing temporarily.
Before bed, I take 2 inches of 3/4 inch wide odorless paper bandage tape for sensitive skin - use anything you can stand to stick over your nose. I cover enough of my nostrils to force mouth breathing (almost 90% for me).
Mouthbreathing is bad for you. It feels icky. Your nose is better at filtering out cold germs. Dry mouth feels icky. However, it is not as bad for you as not sleeping and pain and inflammation that kick off migraines, ear problems, neck and shoulder muscle spasms that kick off spastic colon.
Botox is still experimental and still temporary. It doesn't stop the TMD, the neuromuscular disorder that causes teeth grinding or jaw clenching. If you have grinding or clenching, you need to learn how to stop it.
I'd suggest biofeedback. Basically, a technician hooks you up to sensors and a monitor so that you can view your own muscle reactions in real time to certain stimuli and learn how to control them. Like re-training your muscle memory.
I've only been an observer of biofeedback, not a participant. I worked in a physical therapy office and was familiar with the equipment and the process...but they never strapped me in! I do think it makes a lot of sense, though. Are you considering biofeedback?
jforgizmo, I've never tried biofeedback myself, but I did try the forced mouth breathing for a while. I found it wasn't as helpful as I had expected, because I think I was clenching and grinding in between breathing, and/or pulling enough air through my clenched teeth, so I was still doing damage. Next I tried all kinds of therapies with various dentists and TMD specialists, and finally found a combination that worked: For daytime use, a splint that does not lock my teeth into position, but allows me to move the jaws around relative to one another, and relax the face muscles. For nighttime use, I'm using a splint (called a Bionator) which locks the jaws into position, moves the lower jaw forward, opens up the TMJ's, and also prevents me from grinding my teeth. Now I have no pain or clicking in my TMJs. The specialist was just about ready to give up on me, because we had tried different combinations for daytime and nighttime splinting, and the ones I'm using now were pretty much his only remaining alternatives.
So don't give up on finding the right combination, even if it takes some time.