First and foremost and absolutely most important: “Natural” teen conversation is not about swapping slang. That is, by far, the most common pitfall of newcomers to teen fiction. Consider this exchange of slang: “Dude, that flick was sick.” “I’m sayin’, bro! Way killer.” These are the words of youth, yes, but they sound terrible in written dialogue. Slang will date a book (“groovy”, anyone?), but even worse, it almost always sounds contrived when usurped by adults. Not to mention painful. (Translation for fellow thirty-something-plusers: “sick” is “cool,” not vomitous or snotty.)
Natural teen dialogue isn’t about slang. Rather, it’s about another “s” word: syntax. You want to string your words together in a more footloose fashion, and throw in a little bad grammar while you’re at it. Nothing sounds more contrived in a teen’s mouth than meticulous, proper sentence structure. A teen would not say, “You need to stop doing that” or “Stop running in the hall.” At best, those lines are dull. There’s certainly no youthfulness in them. Instead, a teen would say, “Don’t be doing that” or “Quit with all the running.” You may need to toss a blanket over your signed copy of Elements of Style before attempting this kind of anarchy, but it really will make for more natural teen dialogue.
Another pertinent characteristic of teens is that they are all emotion and reaction, with fewer self-censoring mechanisms than adults. They talk first and think (about both what they say and how they say it) second. Next time you write two teens conversing, let them react and blurt. That way, you’ll have less opportunity to stick a bunch of narrative in between the lines of dialogue describing what the speaker is doing, where he is doing it, and how he is doing it, which chops up even the best dialogue. Sure, you need your narrative pauses in conversation, but be willing to let the dialogue build on itself with a few good back-and-forths before you give readers their narrative breather. It’s a balancing act, I know, but one well worth perfecting. Because when you get this down, the conversations between your teen characters will be more natural—and way sicker.