I'd recommend reading As I Lay Dying by Faulkner. It's the only book I've personally ever read that's done what you're talking about. It is very stream-of-consciousness, so it may not be quite the style you're going after, but it is a very good example of keeping personalities distinct.
Each character should have a unique sets of world-views and prejudices, and primal concerns that color the way they see the world and the people around them. Have their individual speech patterns present in the narration.
You might try writing a single scene or descriptions of something from two or more different viewpoints for practice.