My criteria for keeping a book (and I kept far too many for far too long - getting rid of is a newish thing for me):
"What do I KNOW I want to read again?" and "What do I KNOW I want my kids to read?" If it's not going to get looked at again, why would I keep it? Again, this is a newish concept for me.
I do reread as I get rid of - but I read very quickly, so I can do that and not get bogged down.
I joined paperback swap, though that doesn't really take the numbers down, does it? lol
I find it easier to get rid of books since the GED center started its shelf. (giving to a good cause)