Try to ask questions that will get them to think, rather than simply parroting the WT line.
Silence is your friend.
If there is a long pause between your question and their answer, DON'T fill the silence.
If you have asked a good question, this may be the first time they have actually had to think beyond their programming.
That said, when JWs get backed into a corner with a question they can't or don't want to answer, they are trained to switch topics.
Usually they will hopscotch the Bible trying to get you into one of their prepared presentations.
You almost have to be like a cross-examining attorney and restate your question, insisting that you want to hear their answer.
Be gentle the first time, but if they persist in switching topics or in making comments that don't answer your question, tell them directly that you have to conclude that they are deliberately avoiding giving you a direct answer.
If they say they will have to do research and get back to you, set up a definite time and tell them that you are taking them at their word as Jehovah's Witnesses that they will follow through and come back to you with an answer.