Try using Greg Koukl's technique: Ask questions.
Ex:
Skeptic: "You believe in the Bible? It's just a book full of myths."
A: Oh, what kind of myths?
or
A: Oh, how did you come to that conclusion?
Skeptic: "God doesn't exist."
A: Oh, when you say 'God', what do you mean?
Skeptic: "The Bible isn't reliable, it's been changed over the years."
A: Oh, in what way has it been changed?
Skeptic: "Miracles are impossible."
A: Oh, why do you say that miracles are impossible?
The idea is to keep a dialogue going; plus you're making them do the work (explain why they believe) and you're showing them that you're really interested in what they have to say.
Koukl calls this the "Columbo Tactic", named after an old TV detective that you may or may not remember. There's a short article here:
http://www.str.org/site/News2?page=NewsArticle&id=6879
But better training is available via the "Tactics" series, available on CD, DVD or downloadable mp3 from the STR store, ex:
https://secure2.convio.net/str/site...EW_PRODUCT=true&product_id=4164&store_id=1161
or their ABC training series is also excellent and includes this as well as other stuff, but it's more expensive.
Basically, try to ask questions to probe why people believe certain things. Sometimes they may not know why they believe; at least you'll buy some time to think, and also gain an understanding of why they believe like they do, so that later you can discuss things more with them.