Briefly, the approach is this:
The Watchtower has everyone believing they are living in Judgment Day NOW, and that the judgment revolves around how active they are with the Watchtower, ie. meeting attendance, field service etc.
However, Hebrews 9:27 destroys this myth by clearly saying that judgment occurs AFTER a person dies.
The second myth that the Watchtower teaches is that the New Covenant (also called the blood covenant) is ONLY for government positions, (limited to 144K)
Matthew 2: 27 , 28 destroys this myth by clearly stating that the New Covenant is "for the forgiveness of sins".
So, once the JW at your door admits to you that they are not personally part of the new covenant (they will initially claim that they are until you question them about their involvement in the Lord's Supper), you can simply read these two scriptures from their own bible and the problem immediately becomes apparent.
If the NC is for forgiveness of sins and they are not part of the New Covenant, how do they get their sins forgiven?
If handled with love and concern, you'll get the deer in the headlights look every time.