Continuing on the week's theme of what makes successful dialogue...
One of the things I've seen a lot as an editor and avid reader, both of unpublished and published writers, is a tendency that newer or less-experienced writers have of forgetting all about showing the reader the physical actions of their characters as soon as they begin to speak. That means a lot of exchanges that look like this:
Danielle looked at Jason.
"You really mean that? You're going to ask Sarah to prom?"
"I was thinking about it."
"Why? Because you heard she got breast implants?"
"Don't be mad just because she went through a growth spurt."
Or whatever. Basically, you see lots of dialogue that follows the format of a script or a play: lots of exchange, and not much for a reader to visualize.
The biggest problem here is that you're missing an opportunity--namely, you're missing the opportunity to show us how your character feels about the things he or she is saying and hearing. Possibly he/she feels vehement about the point he/she is trying to make; possibly he/she is lying, or avoiding some other truth, or uncomfortable, or nervous, etc. etc. And the best way of indicating these different attitudes is by showing them.
Danielle stared hard at Jason.
"You really mean that?" Her voice sounded weird and squeaky, even to her own ears. "You're going to ask Sarah to prom?"
Jason shrugged, looking at Danielle sideways. "I was thinking about it," he said, and she wondered when he had gotten so good at sounding casual.
"Why? Because you heard she got breast implants?" She blurted out the words without meaning to, and instantly regretted it. Her face was burning. She had just said the word breast in front of Jason. The last time they'd discussed body parts was when they were three and splashing around naked in the kiddie pool.
"Don't be mad just because she went through a growth spurt," Jason said, chucking her on the arm. She jerked away.
Here you have the opportunity to develop a complex relationship between your characters, and to indicate subtle shifts in what they are thinking and feeling, even within a single conversation. In the first exchange, Danielle's words might communicate that she is jealous, or skeptical, but they communicate little else--you have no idea what her relationship with Jason is really like. In the second exchange, you have the opportunity to learn more about their history together, and their changing relationship through time. Even relatively small physical gestures--he chucks her on the arm (super friendly); she jerks away (the friendliness is grating on her)--can be incredibly revelatory. Is Danielle falling in love with her childhood best friend???
Helpful Hint: When you're editing your MS, go through the pages of text. if you see a lot of pages with a ton of white space, it probably means lots of dialogue that needs fleshing out with physicality and gesture.