It all depends. If I love the book I keep it. The ARCs I usually hold giveaways for them on my blog and the rest I sell to used book stores so I can get more books :)
I'll admit that I am a hoarder and collector of books. They can enter here and then can never leave. Eventually, the books will take over the entire house. :-)
I keep them all until it gets to the point where I'm overflowing. At that point i donate them to my local library. Just this weekend i headed over there w/ three hefty bags full - 70 books ( I counted ). Obviously they love me at my libarary :D
Last night I added a list of books that I will mail to you for free, but there are rules: you've never read the book and/or author before and unless you LOVE it, you have to give it to someone who hasn't read the book and/or author before.
I'm not a massive re-reader so it only tends to be real favourites that I keep on the shelves the rest I list on Bookmooch, so I can swap them for more books to fill their spaces!
You really should check out PaperBackSwap.com! You send a book to a reader who wants yours and in return you get a credit that you use to have a book of your choice sent to you! It's awesome! You can check it out Swap Your Used Books - PaperBackSwap.com">here!
The most expensive choice for getting rid of your books is to grow your own readers. After indoctrinating my sons into reading, in general and scifi/fantasy specifically I have now captured my 10 year old grandson as well. The 3 and 5 year old grandchildren love books but so far they aren't reading any of mine. The 10 year old and I are currently reading the Eragon series and having avid discussions about them. Granted the expense of raising your own readers may be daunting it is by far the most satisfying method of divesting my home of my favorite books. BookMooch and Paperbackswap are less satisfying and way less expensive alternatives.
I have a whole family of readers -- myself, mom, grandma, and daughter. So, any book around here is generally going to get recycled three or four times in my family alone. I would love to know how much I have spent on books for my daughter alone -- or maybe I don't want to know. It can be a very expensive way of life, but I wouldn't change it :)