Tell them, "I'm curious about something. I'm sure you could go on all day about what you personally like about being a JW, but could you tell me three things that you do not like about the Watchtower Society?"
If they can't come up with anything convincing or sincere, you can say "any earthly/human organization is going to have imperfections and if one doesn't feel like they can think or speak critically of their leadership, this just sets up warning flags for me and I couldn't even think about being a part of a group like that."
Another effective technique I heard about was a householder answered the door draped in an American flag. The JWs didn't even bother and bid him farewell, although they did find it amusing.