Should It Stay, Or Should It Go?

I try to only keep the books that I love, or that I want to read again someday.  But sometimes I let books linger on my shelves out of habit...or laziness.  It's hard discarding books.  I live in fear that the book I get rid of today will be the book I'm desperate to read tomorrow.  Which means that I keep a lot of books and that my book shelves are full ... But at least I don't have a growing book hoard creeping across my floor.

I'd like to keep it that way, which means weeding my shelves occasionally.  Never fun, but necessary.  Still, it's not easy choosing which books should stay and which should go.  That's why I try to limit myself to only buying books I love and know I'll want to own forever.  But bookstores are temptresses, and somehow I always come home with more books than I should.

So, what's your criteria for keeping or getting rid of a book?  How do you choose which ones should stay ... and which should go?