#1 Choose a Voice -- I've said this many times, but it's a very good idea to find the person you want to play a role or even to "inhabit" a part in your novel. Basing a character on a real person with a voice all his/her own enables you to keep it clear in your mind how he or she might say something. Now that is not to suggest you can't create upon their basic voice and be creative, but it's so helpful to enable you to hear it in your head. I don't know about other authors, but I can very clearly hear the voice in my mind. Even when I email people back and forth, I literally hear their voices as I read. This innate ability allows me to write characters with that voice "imprint" in my head to make it real.
#2 Read it aloud -- Read the dialogue out loud. How does it sound? Clunky or smooth? Does it sound like how someone would really talk? If you can't manage to read a passage of dialogue without stumbling and stammering, it's not good dialogue. For the record, most people don't talk in long-winded sentences. Unless your characters are having a true discussion then they are probably bantering back in forth in short clips of dialogue.
Dick, Jane, Dick, Jane -- Names we don't use. Overuse of the character's name drives me crazy when I read dialogue. How often do you really say a person's name in person? In my house, we rarely call each other by name unless we're trying to emphasize something. Don't overuse the person's name unless you're trying to emphasize or direct something dire or important at them. It's an annoying redundancy your audience doesn't need to hear over and over and over.
Narrative as dialogue -- a quick indicator that a writer doesn't understand dialogue is when you see him or her use it as a narrative tool. Your characters should not be narrating the action unless they are literally narrating the story. First, narrative as dialogue doesn't sound natural or real. It often is redundant, as it's repeating something we are about to see or we just saw.
Backstory in dialogue is an artform -- Be careful how you put backstory into dialogue. Again, it can't sound forced or unnatural to the scene. If you need to explain backstory, put it in the narrative (that would be my first choice). If you need the backstory to inform the characters in the present, put as a part of a natural "getting-to-know-you" scene. Don't force it in or it will undoubtedly sound like you did, in fact, force it.
Below is a scene from California Girl Chronicles: Brea's Big Break. Before I show you the scene, let me point out what the dialogue did in effect do, and you don't notice it. Letty essentially gives us important backstory in a natural way. She tells us the following: Drew left the bikini shop after Brea quit; she too had a brief fling with him; she thinks he did love Brea (which book one suggested); and that Brea was hurt by this affair. Why does this matter? Book two has to somehow suggest to new readers what was going on in book one and what happened afterwards. This conversation covers those two bases in a non-distracting way. See if you agree.
About 30 minutes later, we found ourselves sitting on barstools, eating stale tiny pretzels and drinking shots of Patron. I had two shots and that was quite enough to catch a zing of a buzz. Letty had three shots, and she was drunk and happy. She giggled her way through the conversation, which eventually led to questions about Drew’s fate. She said he quit the day after I left.
“Whore!” she crowed. “He left me! It’s all your fault.” She laughed. She threw a pretzel and hit my forehead.
I began laughing. “Sorry! But you didn’t lose much,” I replied and threw a pretzel back.
“Oh, fuck him,” she cried. “You know he told me he fucked you … twice!” she started cackling.
In the past, this announcement would have made me angry. Now, I was just disgusted and not surprised. “I see, he kissed and told,” I replied.
Letty leaned in with a grin. “I fucked him once in the back room,” she admitted and laughed. “It was okay,” she added.
“Supply closets,” I muttered in disgust. “Well, he fucked many,” I said as I lifted my glass. “Cheers!”
Letty obliged back, and our glasses made a “clink” as they purposefully collided. Letty settled down a moment and became thoughtful. “I think he really loved you, Brea.”
I glanced over at her and considered that suggestion. “Maybe,” I replied. “He hurt me though, and that’s enough of that.”