Stacey Barney, senior editor at Penguin/Putnam for Young Readers gives advice on dialogue:
• #1 rule of dialogue: Ordinary must be extraordinary.
• Characterization must come through. Must stay true to the character.
• Dialogue is not pushing the plot, it's enhancing it by drawing out and heightening characterization.
• The only true goal of a good character work is to develop characterization.
• If you take out the dialogue and the story doesn't make sense, then you need to rethink your dialogue.
• Accents--suggest the accent, don't overdo it. Use it sparingly. Stay true to the language used in a certain region. For example; ya'll and dropping g's on the end of a word so the reader doesn't get frustrated.
• Dialogue tags: Don't overdo 'he said', 'she said'. Make sure the setting is clear at the beginning of the scene as a point of reference so the reader knows where the character is. You don't want to rely to heavily on dialogue tags.
Each bit of dialogue should have a purpose.