How do you organize your shelves?

Who knew getting a new bookshelf would make me think so hard. I can't decide how to organize my books now: alphabetical by author, by size (since the shelves are graduated), or by genre. Plus I kind of like the idea of having world literature organized by either country or continent. Decisions, decisions...

How do you prefer to organize yours? (I've asked this once before in a poll and the winner came out as Genre)