My first time out, two other agents went with me and after the first couple houses we were pretty comfortable and split the duty and hit the houses separately. All was going well, and then I got to THE house.
The man cracked the door, and I started my introduction. He promptly said, "I don't need no d*mn real estate!" Then he drew the door back until the hinges buckled a bit as to get a full windup to slam the door so hard all the windows on the house rattled.
It only took about a second for me to fall into a ball on his front step laughing my a*s off! It helped that my fellow agents were laughing, too. Had I been by myself I might have slinked back to my car with my feelings hurt!
Best thing to help getting in the mindest for door knocking is the set of tapes from David Knox "Take the BO out of FSBO." Keep up the knocking, and laugh the crazies off.