Once Upon a Time

How does your story begin? Does it begin with, "Once upon a time?" or in media res? How much back story should you put in, before cutting to the chase? Personally, I like beginning in the middle of the action. I think this draws the reader in. However, I have read novels that begin prior to the action and I like them just as well. I guess it all depends on you as a writer. What feels right to you? Even if you begin with the back story, you can always take it out later. The author needs to know where his/her characters have been as well as where they are now and eventually where they will end up. This does not always happen in this order. Oh, were it so that writing was so black and white. Often, at least for me, my characters hijack my story. I may have had a specific goal in mind when I began putting fingers to keyboard, but this changes rapidly depending on my characters and what they want. I feel I am channeling them, sometimes to my consternation. For instance, I once set out to write a ghost story about The Y Bridge for an anthology my writers group was putting together. I put down my characters name, Amelia and she had other ideas about the story. It turned out to be a wedding story, a piece of chick lit and so unlike what I normally write. So, where do you begin? And more importantly, how does this help where you end?