Scott, I have been told that a little dab of high temp silicon gasket goop works. I use some goop called "forme-maker" or "gasket eliminator". It is a ford motor company product that comes in a small tube and looks like a thick grape or strawberry jelly. I've been told that it was designed for the installation of sensors on the intake manifold. It only takes a very small amount on clean threads to work. It never hardens completely so it can be removed but it still keeps the muffler/manifold from loosening or leaking.
Fred