I get rid of them. My forcing function is that I won't buy any more shelves - I have two bookcases with about 4 shelves each, shared with my girlfriend, and if I want to keep a book I have to get rid of one. It's still probably 300 lbs of books but it keeps it bounded. (And a lot of those are hers, because she went to law school and still has many of her law books. Most of mine are design or cooking, since the color plates don't really have electronic equivalents yet and some are signed.)
Also, for text-only, transient stuff like business books and history books (that I'm unlikely to read twice), I buy a lot of Kindle stuff now.
Also, for text-only, transient stuff like business books and history books (that I'm unlikely to read twice), I buy a lot of Kindle stuff now.