I keep them all, but try not to have more than a years worth of unread books to be read.  If it starts to get out of hand, I take time out from other activities and do some dedicated reading.  I've only every thrown away one book, a history book that was so poorly conceived that I tossed it in the nearest trash bin two-thirds of the way through reading it.

In terms of the future of my collection (nearly 1000 SciFi and Fantasy books) I am trying to develop an interest in these genres in my niece and nephew.  Hopefull one or both will want these books.