1. Listen to people when they're talking. Most people use contractions unless they are giving a presentation or are particularly scholarly or proper.
2. Make sure the voice of your characters doesn't change over time unless there is a reason for it changing over time. For example, if someone doesn't use a lot of profanity or slang, they shouldn't suddenly start using it for no reason, but they could start using it if they are hanging around a bad crowd for example.
3. Make sure to use enough speech tags that your readers don't lose track of who's speaking, but don't use too many and don't make them too unique. What I mean to say is that he said, she said are usually fine. When you get into, she exclaimed, he trumpeted, this is going to take your reader out of the conversation. It should be clear from context whether your character is whispering or shouting.
4. Make sure to keep a character's speech with their action. For example, Miri came and sat down next to me. "So what are we doing today?" she asked.: Yes. Miri is sitting and speaking in the same paragraph. But, Miri came and sat down next to me. "So what are we doing today?" I asked.: No. The question now needs to be in a separate paragraph because Miri is performing an action, but someone else is speaking.
Hope these pointers are helpful.
I Write, I Edit, I Write Again. Witness!
We're Making Better Words, All of Them, Better Words.
I Write to Burn Off the Crazy.
A Good Day Writing is a Day Writing.
It Puts the Words on the Page or It Gets the Hose Again.
Just Keep Writing...Just Keep Writing...Writing, Writing, Writing!
Writing is Magic!
The First Rule of Write Club is You Talk About Write Club.
If You Aren't Writing, You Aren't a Writer.