I've been running my old paper books through a scanner and throwing them away. The resulting pdf's are readable on the Kindle or my computer, or even on my ipod. I've probably done around 500 books by now. There's a looong way to go yet :-) I don't see much point in having physical books anymore other than coffee table books. I like having my library at my fingertips rather than scattered everywhere.
And by throwing away you mean "donating them", right? :)