This past Sunday, I drove "all the way down the mountain" to Colorado Springs to hear literary agent Rachelle Gardner speak on openings. I'm going to hit some of the high points in this post.
Since I'm starting a new book, I'm facing that dilemma, and it doesn't get any easier. And, since Hubster read the first scene of my efforts and said, "I don't like the first line," I figured it couldn't hurt to have a 'refresher' course in what an opening should do.

She gave us some excellent reasons why openings are so hard to write. The first, that as the author, you've probably got an emotional attachment to your opening scene because you've envisioned it for days, weeks, months or years. But it might not be the best way into your story.
Or, you don't really now where they story begins.
Or, you don't have a true feel for the theme or premise until you've finished the book.
(For the record, the second one fits me best.)