At The Write Practice, Joe Bunting offers not only sound writing advice; he offers the opportunity to practice whatever skill he’s exploring on a given day. This article, 16 Observations About Real Dialogue, is one of the best I’ve read. Very practical.
For example, here’s Observation # 5:
5. Real People Refuse to Repeat Themselves
Sometimes, when the other person can’t hear and says, “Huh? What did you say?” real people don’t repeat themselves. They say, “Nothing. It’s not important. Never mind. I’ll tell you later. Forget it.”
Sometimes, this leads to bickering.
This technique is especially effective if a character has just said something vulnerable. People will rarely repeat something embarrassing or hurtful or vulgar. You can draw attention to their vulnerability by having them refuse to repeat themselves.
Visit The Write Practice, a great resource for both beginning and more experienced writers.