Good question, and one I asked recently on another board. I didn't get any suggestions I could really use, but decided that it's not really all that needed anyhow.
You know the truth, and should be prepared to say it. Don't ever be afraid to say "I don't know" when faced with a question you don't know the answer to, although I'd suggest adding "but I'll check it out and let's get back together to discuss it".
One thing to keep in mind is that you can know everything about what's wrong, but if you don't know what's right you're still in a mess and won't be able to put up any decent type of defense.
I've used this analogy in a few different situations, and it works for this one too. It goes...
In banks tellers must learn to tell real money from counterfeit money. In order to do this, you'd think they'd be introduced to counterfeit money, made to examine it closely in many forms. However, that's not how it works. Tellers are given real money. They handle it, examine it, and deal with it closely. The reasoning is that once you've learned what real money is, you'll be able to tell a counterfeit bill at a glance.
It seems the same is true in Christianity. If you know the truth, you won't be deceived by untruth.
And btw, no matter how much you think you know, don't get superconfident. Pray before the meeting, pray with the Witnesses before you start your discussion, and pray in your mind during it. You never know when you'll get thrown for a loop, and usually the person knocking at your door is not someone who hasn't heard your objections and arguments a number of times in the past.
Chrys