Hang a I LOVE RAY FRANZ sign on your door. This should effectively get rid of 99% of them.
Actually, I have a tactic similar to this, but haven't yet had a good opportunity to use it. It depends on the individual at the door. If they seem nice, I am very polite in return but then inform them that I DAed about 12 years ago and to please put me on their do not call list.
If they aren't so nice, I have a backup plan. I keep a my Crisis of Conscience near the front door. I'll say, "Oh, you're using your book? Let me get mine!"