I'm not a teacher but it happened to me to have an "audio book" called
Street Talk. It's basically a recording of dialogues with american slangs and idioms.
First they talk using the standard English and right after they show you a recording using slangs.
Personally I'm not very interested in American slangs but I have to say that this audiobook was very pleasant to listen to and a not painful way of learning informal English. Plus you can learn English intonation and pronunciation because this audiobook is not monotone like most audio materials.
I found the amazon link:
Amazon.com: Street Talk-1: How to Speak and Understand American Slang: David Burke: Books