A couple of options that have worked for me:
1) Draw a chalk outline of a body on your porch. Leave a blood smeared briefcase and a torn copy of Watchtower next to it. Keep a scoreboard running like the HardRock Cafe uses: "Jehovah's witnesses killed this week: 17"
2) Immediately upon opening the door, don't give them time to start. Jump right in and begin trying to convert them to another religion. Scientology is a good choice. Don't pause for breath as you'll lose momentum. Just hammer away and make sure to keep asking for money so they can get to a higher level. They'll be out of there so fast they'll leave skid marks next to the chalk outline.
3) Land mines
4) Try selling them life insurance. Something like "Sure, the kingdom of Heaven when you die is nice, but let's talk about your survivors. Have you considered term life to meet their needs?" Invite them in tp talk about their needs and what they can do to guarantee their families financial well-being in the event of their untimely demise. Keep focusing on the "untimely demise" part and glance towards your collection of kitchen knives when you say it.
5) Answer the door completely naked. Or, even better, naked and aroused.