1) Dialogue Tags—Use these only when necessary (for example, to show a change in style of speech or emotion, or to identify the speaker in a group situation). At other times, let the dialogue speak for itself, or have the character commit an action either immediately before or after, rather than using a tag.
2) Real Conversations are Boring—If we wrote dialogue along the lines of real conversations, it would be exceptionally dull. Discard most of the pleasantries and hesitations, and cut to the chase. Make sure all dialogue is relevant to the unfolding story.
3) Natural Speech—Dialogue in fiction needs to sound natural even though it isn’t. One way to achieve this is to use contractions and fragments. Even the best of us don’t speak 100% correctly all of the time. We’re more likely to let grammar slide in speaking than in writing.
4) Quirks of Speech—Certain ‘catchphrases’ can work well for characters; however, don’t overdo it as it can also become repetitive or descend into caricature.
5) Accents—Caricature is also a potential risk when dealing with accents. There’s nothing wrong with showing an accent phonetically, but don’t make it so heavy the sentence becomes unintelligible, or comes across as a cultural stereotype. Choose one (two at most) aspects to show (e.g. a dropped H) and leave it at that.