The easiest way into a Christian's head is to start comparing how they act with how they believe. It is hard to do this without making it personal, but with practice and a heaping dose of respect for how much it hurts to hear the charges you can do it.

I've tried this when arguing with utilitarians. As far as I know, it hasn't persuaded anyone.

Also, it may be possible to discuss materialism as a counter-theory without trigging the massive defense system people generally carry around with them. If they turtle up, just walk away or use them as practice for defending your own beliefs.

Instead of "defending" your beliefs, you may want to try exposing those parts which seem weakest, so as to increase the probability that any errors in your web of belief will be be brought to your attention by your interlocutor.