• Don't get wild with attribution lines.
USE he said, she said / she asked, he asked
NOT she explained, he blurted
It's not that you can't use the latter, but if they are overused, they take away from the dialogue. Also, sometimes new writers use a word in place of said that isn't even possible. Have you ever laughed or chortled words? Said and asked don't slow the reader down. Readers are used to overlooking them.
• Avoid adverbs in your attribution lines.
Again it's not that you can't use them, but your dialogue should be clear enough that for the most part adverbs are unnecessary.
• No talking heads.
Dialogue should not be in a vacuum. Readers need some sense of setting--some idea of what is around the speakers and/or of what they are doing, which leads into the next point.
• Use tag lines which incorporate action.
Brian leaned against the front door.
Sarah threw her backpack onto the couch.
These replace the said or asked. Don't combine them, i.e. Brian said as he leaned against the front door, or Sarah asked as she threw her backpack onto the couch. The word "as" is a warning signal that you may be doing this.
• Make each character sound unique.
Is she wordy? Or does she use short tight sentences? What's his pet word? Is the language age appropriate? Is all his talk serious or hardly ever serious? Often, people's speech is not grammatically correct. One trick to check the sound is to highlight each character's speech in different colors. Then go through and just read one color. Does it sound like the same person? If anyone could say all of it, it isn't unique enough.
• Avoid or limit dialect.
Don't make your readers have to guess what the characters are saying. Yes, I know many older books did it, but readers today aren't as patient.
• Avoid filler words.
You don't need uh, um, well, etc. Often you can skip greetings and parting phrases, too, as they don't really add.
• Keep it age appropriate for realism.
A five-year-old saying, "I think it would be beneficial if I had an animal companion of my own," would not be believable. But how about, "Can I keep him? I don't have a pet." Yes, that's more realistic. This may seem obvious, but often beginning writers use adult language with child characters.
• Dialogue should put readers on scene.
It should make your readers feel they are there listening and watching these characters, yet not be like a recording of an actual conversation. We don't need to know all the inconsequential things that people often say in real life. "How are you?" "Fine."
• Dialogue should move the story forward.
No lectures. No here's all the info you as a reader need to know about this character. It should not contain content that no person would say to another, i.e. "Remember how my mom died when I was a baby and I was raised by my aunt?" Another warning sign is when one person's speech goes on a really long time.
• Add internal dialogue.
The main character is the one whose internal dialogue we are usually privileged to hear. It won't be in quotes. Some publishers use italics; if it helps you to see the thoughts, go for it. Sometimes the aid of "he thought" or "I thought," is necessary, but not always. The contrast between spoken dialogue and internal dialogue can really make your character come alive. Often we don't say out loud what we really think. Letting your readers get a glimpse of that will add interest to your story.
• Start a new paragraph when the speaker changes.
This helps signal readers that someone else is talking.
Using the above tips, takes this:
"Molly," Trevor exclaimed impatiently. "Where are your gloves?"
"Um, I don't know, Twevor," Molly lisped softly.
"Well, you can't go outside without them," Trevor complained loudly. "Your fingers will freeze and Mom will blame me."
"But I want to go," Molly whined. "I want to slide."
"Sled," Trevor corrected.
Trevor glanced out the entryway window at the falling snow. If I didn't have to wait for an annoying little sister, I'd already be flying down Hawkins Hill. He sighed, and knelt to zip up Molly's coat. "Where are your gloves?"
Trevor frowned. "You can't go outside without them. If your fingers freeze, it's me Mom'll yell at."
"But I wanna go," Molly said. "I wanna slide."
"Sled," Trevor corrected. He dug through the heap of clothing on the closet floor.
See how we know Trevor is impatient? Trevor more realistically talks about Mom yelling instead of Mom blaming him. We get some setting and some action, and know that Molly is a lot younger.
Finally, listen to kids talk.
Your own are good, but even better are kids you don't know. Go to a public place and pay attention to the children or young adults talking. Not only can you hear the rhythm of their speech, but you'll be reminded of what they are interested in. Taking notes on what they say--not what they look like--can help you practice dialogue.