In a New York Times interview about the late writer/director John Hughes, Molly Ringwald ("Sixteen Candles," "The Breakfast Club") said this about his dialogue:
"So many scripts I would read – and I still find this to be the case – whenever there was a teenager, it just does not sound remotely like a teenager. It sounds like it’s written by an adult for a kid. And then the kid actor is trying themselvessound like a kid, and they just don’t. For some reason, when I read John’s dialogue, it was totally unique. It was like his own language. But it made sense. I didn’t necessarily hear anybody that was talking like that. But we could."
I think the secret of good dialogue is in there. It doesn't try to ape the way that kind of person talks, but when you hear it, it makes sense.
Easier said than done, of course. But it suggests a direction: listen to the kinds of people you want to write about. Let them get into your head. And when it comes time to write their dialogue, don't consciously try to fit in jargon they would use, just let it come out through the filter of your mind. (Also, it couldn't hurt to go back to some of Hughes' films and listen as well as watch to see how he did it.)
(There's lots more about characters and dialogue in my book, "Your Writing Coach," available from Amazon and other online and offline retailers. If you're interested in writing scripts, go to www.ScreenWritingSuccess.com for lots more tips and sign up for my ScreenWriting Success newsletter there.)