I encourage people not to either verbally or physically harass JWs when they come knocking on the door.
Saying this, I recognize that it is ever so tempting to try the following:
1) Answering the door wearing a T-shirt with a logo such as "Satan Rules", "Member of the Evil Slave Class", and "1914, 1918, 1925, 1975, What's Next?"
2) Answering the door wearing only underwear (or less).
3) Dropping a name of some governing body member and suggesting that you got to know him when he and you were in jail together, and then ask if they know if he's stopped some particular filthy habit.
4) Answering the door while brandishing a pitchfork.
5) Say that you're so glad they came and that it's good for people to have an open mind about their religion and to be willing to change; once you get them excited with this, show them a copy of the WTBTS failed prophecy list (backed by quotes from their own literature) and ask if they are open minded as well.
6) As most JWs have never looked at a copy of _Studies in the Scriptures_, tell them that you've gotten many of your religious beliefs by pyramidology; then show them a copy of _SitS_ and point to the WTBTS copyright notice.
I have #5 and #6 ready for the next visit.