One of the most effective techniques I observed for getting rid of any door-to-door-missionary (be they JW, LDS etc..) took place while I was staying in the home of a Methodist minister.

They would knock on the door, and he would answer and politely invite them in, and make them herbal tea. He would then reach over to the small table next to his chair and grab his bible and a notepad. He would take notes as they talked, but say next to nothing. After they'd finished the standard speech he would go over his notes, open his bible and point out to them passage by passage how wrong they were (this, of course, according to the Methodist interpretation of the scriptures). he would then send them on their way.

From what I have heard his home is now "off limits" to all JW and Mormon Missionaries.