Keeping them focused on one topic is the key in my humble opinion. It's a tactic that isn't talked about enough on this board.
They so easily divert from the argument that it is hard to keep focused - I know personally.
I heard Dr. Walter Martin - in a mock dialogue with a JW do the following when a JW tried to divert (paraphrased from memory):
JW: "So Mr. Martin, you are saying the Society altered the greek a little bit. But we are the only religion that do not go to war."
Dr. Martin: "Wait. War? Who said anything about war? So are we done talking about the inaccuracy of the New World Translation? Are you admitting that the JWs have added to scripture where it suited them?"
JW: "Well ... no ... but ..."
Dr. Martin: "Okay then, let me show you another scripture and how it is rendered in the original Greek. Then let's look at your bible and see how the Society rendered it."
Problem is, for most of us, we might be tempted to take on the war argument and go in a completely different direction. Then they can use their vague reasoning and a couple scriptures out of context and then you have to try and untangle THAT mess as well. Better to do like Dr Martin and take control of the conversation and make them follow their entire path of logic.
If you're not a bible thumper then you can use any number of other topics, like evolution or archeology. And just stick to your guns.
Priceless!
-ithinkisee