What words do your characters use to express themselves? Their choices say something about their personalities, age, culture, education, outlook on life, etc. Every character should say things that reflect his or her individuality. Your characters should not all sound the same.
CASUAL TALK or NOT: Do they say Yeah, Yes, or Yep? Gonna or Going to? Don’tcha or Don’t you?
EDUCATION and PROPER GRAMMAR: Do they never split their infinitives? (to really sound educated vs. to sound really educated). Do they say "I’ve got to get some breakfast" rather than "I have to get some breakfast"?
EMOTIONS: When they’re impressed, do they say "Awesome!" or "Fan-freaking-tastic!" or do they use a made up word from their culture, genre, and times—like Firefly’s “SHINY!”
SWEARING: Do they curse like a sailor and throw out F-bombs, or do they erupt with a mild "Oh, for Pete’s sake!" when they’re peeved? In my near-future sci-fi novel, The Body Institute, Morgan’s exclamation of irritation is: “What the haze?” Some characters might not even wait until an annoying moment to pepper their speech; like people in real life, their swearing is a part of their everyday sentences.
EXPRESSIONS, etc: Do they throw out silly words like Zoinks and Yikes and Yoo-hoo? Do they make up words like yummers, nerdify, germ-ful—or brillig and slithy toves (thanks, Lewis Carroll)?
VOCABULARY: When speaking, do these people choose complex words or simpler ones? Such as compulsory vs. required, insubordinate vs. naughty, docile vs. quiet, hullabaloo vs. commotion—or even simpler, fuss?
CULTURE or ETHNIC INFLUENCES: What your characters have grown up with, such as You guys or Ya’ll. Or Crikey. Whether the third meal of the day is labeled Supper or Dinner. Whether you’re referring to a British Biscuit as opposed to a U.S. Cookie.
AGE and SLANG: Words come and go, so be careful if you’re older and writing for tweens or tweens. Especially when it comes to slang. "What a drag" and "bummer" and "lame" are apparently not used much anymore, but if you have an older character—perfect! Use those phrases for them. Expressions like "Are you pulling my leg?" and "That’s SICK!" may fast become obsolete and change meaning by the time your book is published.
I’ve heard it said—and I’m not sure how successful I am about this in my own writing—but you should be able to read a scene in your writing without using the dialogue tags (those things that indicate who’s saying each line, e.g. he said), and be able to tell who each character is. They should sound THAT different.
How do YOUR characters express themselves? Casually, oh-so-proper grammar, or what?
What do you use for slang in teen novels—do you try to stay updated, or do you steer clear?
Have you ever tried reading your scenes with dialogue ONLY, to see if your characters have distinct voices or ways of talking?