Library book sales. They are the greatest thing ever. Books in Good to Fair quality (though some have definately seen better days) for cheaper than book resale shops.
The great thing about the one here in Houston is that on Sunday, you get grocery bags and can stuff them as full as possible and only pay 10 bucks for it. So if you know how to properly arrange books in a paper sack, you can get close to fifty paperbacks for twenty dollars.
Heh heh, I actually work at a library! Thing is, I tend to have an aversion to used books, unless they are in impeccable condition and in a subject I'm interested in.