Authentic dialogue often doesn't mimic what's on TV or in the movies. In real life, when a couple is calling it quits, the guy wouldn't say, "We'll always have the wall behind the bleachers."
Do you eavesdrop? When was the last time you've ever eavesdropped, intentionally or unintentionally, and what did you take away from the conversation?
Many writers and I agree that an effective way to grasp authentic dialogue is to eavesdrop on real conversations. Friends and family, you're safe- I don't listen in on conversations of people I actually know. I eavesdrop on strangers. When I'm out in public and catch tidbits of conversation around me, I listen.
I don't need to go out of my way to seek out random conversation. Random conversation often comes to me.
It came to me when I was sitting near a couple at a restaurant and the woman was telling the man why she wanted to break up: You don't ever @!#$ take me seriously. You have failed to- let me finish- you have failed to- see, there you go interrupting me again.
It came to me sitting in a school study hall, where a boy explained to his friend why he couldn't get a date: It's not like I can't get with someone, but there's no selection at this place.
And it came to me on a plane as two college-aged girls sitting behind me gossiped about another girl: And she came up to me and said she liked my dress- the one I got from Neiman Marcus- and it was totally weird the way she said it, like she was hitting on me.
Sometimes the people I eavesdrop on are pleasant, and sometimes, as you can see, they are not. I don't write down any conversations I eavesdrop to. I just absorb pieces of the dialogue and trust my memory to help me rehash the dynamics of certain kinds of dialogue when I need to use them in my writing.