I respond to the doorbell or a knock on the door with a loud and gruff "Who is it?".

I've found this is usually successful in intimidating anyone coming to my door. The point is to throw any potential evil-doers back onto their heels long enough to get the advantage.

One day about 2 months ago I did this and heard a little squeak for a response. This intrigued me enough to answer the door. In front of me sttod 2 very young little girls selling some homemade craft items with eyes as big as saucers. I felt horrible!