I don't have JWs come to my door, as they all know me and pass us by when they work our street...
But I was visiting a friend one time and the dubs came knocking. I answered the door and was ready to rumble with them (not letting them know I was an inactive JW), but they were actually pleasant and seemed sincere so I immediately changed my mind. They're misguided just as we were when we were in. Why take it out on them? Plus, if you get in a raging debate you're only going to strengthen their resolve that they are doing the right thing.
I think it's better to be polite, let them get through their semon and then pick one subject that you can draw out of them and, without arguing or being uppity, point out the error of the doctrine or belief and allow them to try to defend it. By the time they leave, you will not have won anything right off...but, it may just plant a seed of question or doubt. It might make them look it up, if only to prove themselves right...but could shake up their faith in the teaching if they see the same evidences that we saw.
When the 2 dubs finished their presentation, I poliltely told them that I wasn't interested in joining their religion but that I had read their literature and mentioned that I had a problem with one of the main tenants of the religion. The brother asked what it was and I said that according to Witness teaching, Jerusalem was destroyed in 607BCE and that their belief that Jesus returned invisibly in 1914 was tied to that date. But unfortunately, there is no evidence that points to the date of 607, but actually 586/587. If the destruction date that the Witnesses project is incorrect, then their belief in Jesus return is incorrect. I didn't say it in an accusotory way or to incite an argument, but stated it as fact. The brother in the lead was a bit taken back, I think, because he probably couldn't explain 1914 anyway.
All I hoped to do was to plant that seed. Maybe he'll go back and research it. Maybe he'll look up some secular references on Jerusalem. Maybe, just maybe, it was the start to helping a person release the shackles of slavery to the Society.
Instead of treating them like the enemy, why not view them as slaves or prisoners that need releasing and we have the means to help them?