Answered a forum post on this and decided to share it with you all.
First off, giving each character their own unique dialogue/mannerisms can be done afterwards as an edit. So if it's not perfect, don't stress.
Second. I find writing a script helps. With narration stripped away you can better gauge how a conversation flows, whether the rhythm of speech repeats too much, and how each character's voice stands out. Ideally, you as the author should be able to tell who's speaking just from the dialogue.
Try listing every way something can be said, then picking which suits the character most.
Jump:
:pointr: Dialogue
:pointr: Body Language