James Bell makes a few points about dialogue, mind you these are in no way in depth, in an overview chapter at the beginning of the book. Dialogue should be unique to each character (they should sound different) and they should tell us something about each character. This seems obvious as each person has a unique way of speaking. it is difficult to do in practice, however. Something that must be overcome is your own style of speaking dominating the characters in your book. Of course, if your voice is coming through, then in an general way the language of your characters will be influenced. Giving each of your characters their own personality through their own voice is a matter of knowing them and overcoming your style of speaking.
I have one character, who borrows heavily from me, and he speaks like I do. Oddly enough this does not make it easier to write. He becomes more difficult to write because each scene becomes a "what would I say" scene. I have another character, Ng, she speaks in her own unique way, all words clearly pronounced. Ng uses no contractions, it is hard as hell to write like that.
How do other people make their dialogue unique?
Also, how do you surround your dialogue? I use a mix of she said or he leaned back and closed his eyes or nothing. It is a tricky thing to balance and I tend to favor using only enough to show the readers who is speaking.