Post NO SOLICITATION, and be aware of your neighborhood's solicitation laws. I know that UA is very strict with door-to-door: you need to buy a 90-day permit, only certain calling hours, and UA give out free NO SALES stickers you can put on your front door. Any violators can get a nice conversation with a cop if you so choose.
I answer the door, but my wife won't even go there. She's been at the kitchen sink, looking out the window eye-to-eye with someone at the door and she won't answer.