I'm finding that if you ask a jw a question as though you are completely confused (its got to be "sincere", not aloof), dumb, pondering, unsure, unconfident (that's the biggie), they will listen to what you have to say. If you ask things with even a *hint* of cocky attitude or a 'let me show YOU what you don't know' attitute, their minds will shut off instantly. Just speaking from experience. ;-)
Although they won't agree with you immediately, at least you know it's in their brains. So when they lie from the platform about the churches of "christendom" and their tithing policies, your jw loved one will have that split moment when they realize the jw's do the exact same thing, even if they don't call it tithing.