2. According to size and colour.
It took me almost a month to get around to colour-coordinating the bookshelf. As it turns out there are just far too many whites and blacks in my collection; not to mention, some books have spines so battered, it’s impossible to guess what colour they are. Still, I’m happy with the result. (I borrowed the basic idea from here.)
Do you have any other ideas?
How do you organize your shelf: according to author, publication, genre?