I would like to start with some more technical questions:
* How do you format the text when you write your dialogues?
* What grammar rules are there to follow when it comes to english when writing dialogue?
Now... to the big question:
What are your Do's and Don'ts when it comes to writing dialogue?
Edit: Yes I know that there is a do's and don'ts thread regarding writing for games, I am asking for a more specific discussion here.
When you say you want more specific do's and don'ts, can you be more specific as to what's missing from the existing do's and don'ts thread?
Formatting of dialogue text:
Use Courier (or Courier New) font.
Each line (or each continuous scene) needs to be preceded with an asset name.
For each line (or continuous scene), you have to precede it with a description of how it's used in gameplay.
Use normal grammar rules, or use grammar rules that are appropriate for the speaking character.