Story Told from Then, Story Told from Now--Getting Clear about Your Narrative Point of View

Deciding who is telling your story--that's a big moment in writing a book.  But even more important is deciding where your narrator will be standing, as he or she tells the tale.  Is the narrator speaking in real time, as the story is happening?  Or from what's called the "retrospective" point of view, looking back from the distance of years?

Which narrative point of view will best serve your story?  Can you move back and forth between them?  And if so, how do you weave them together to make a cohesive book?