Here are nine points taken from the first two chapters of The Fiction Writer's Guide to Dialogue. The book has much more to be said about these topics–when to use them and when not to use them properly.
adverb (loudly, quietly)
italics of what was said
"help me" = emphasis, loudly, or shrill
use ! sparingly
brevity & brisk dialogue
1-3 sentences compact, every word has meaning
by 6 sentences, the other character becomes a listener, and drama shifts to the speech
limit stating the obvious
like when is saying "It's a beautiful day." not obvious? When things are horrific and not beautiful like in the middle of a battle in war.
omit most greetings and salutations
most do not reveal character nor move the story along
only repeat what was said to drive a point home
limit ah, um, and hmm