Before we moved I had a book sale and sold about 700 books. I made a lot of money!!!
I agree with the other posters, unless it is something me or my kids absolutely love and read over and over again, I get rid of it.
I also got rid of most non-fiction, reference type books. If you can find the answer on the Internet then I get rid of the book. I don't need a book on ants for my kids. They can look it up!
I also get a lot of books on kindle now. I am down to only two bookshelves and most of those are favorite picture books and a few series I waiting for my kids to finish (then they are gone too).