Meme- Weeding

This question comes from Booking Through Thursday, and it's rather long, so I've just included highlights here (or what I want to answer):

When’s the last time you weeded out your library? Do you regularly keep it pared down to your reading essentials? Or does it blossom into something out of control...?

Or do you simply not get rid of books? At all?

And–when you DO weed out books from your collection …what do you do with them? Throw them away (gasp)? Donate them to a charity or used bookstore? SELL them to a used bookstore? Trade them on Paperback Book Swap or some other exchange program?

The last (and pretty much only) time I seriously weeded my library was when we moved from California to Virginia. We had a volkswagon type van and filled it to the brim with all the stuff we could carry, and drove it across the country. There was little room (or weight capacity) for all my books. I left half of them behind (a few hundred). It really hurt to cull them all out! Once we found a permanent place here I settled all my books on the shelves, and after a while started missing many of those left behind. Over the past few years I've gradually found new copies of many of them. If I run out of shelf space I might start getting a little more picky, or ferret out those books I don't truly love, but for now I'm not putting a cap on it. Dangerous, I know!

Only two times ever have I thrown a book away. Both had come to me used. One had such an awful odor I felt sick every time I tried to read it. Trash can. The other had some pages stuck together with gum, no way I could get that apart and still read it. That book got composted into my garden.

Books that I don't want to keep- that I'm not enthralled with or don't see myself reading again- either get traded on Paperback Swap or Book Mooch. Sometimes I make up a box and send to Powell's for credit so I can buy more hard-to-find books I really want from them. (I used to trade books in for credit at used bookstores in person, but there aren't any close to where I live now). At my local community center they have a book trade every month, I take a lot of books there and look for new ones. And the last time I traveled to my in-laws I left a book behind that I read on the trip, decided not to keep and found someone who wanted to read it. One route or another, the books I don't want find their way into the hands of new readers.

What about you? Do you regularly cull through your book collection, or only (like me) in dire straits? What happens to the books you decide not to keep?