I joined LibraryThing last year, and finally finished entering books a few months ago. As I went along, I tagged the ones I haven't yet read, and thus discovered that I have far more of these than I thought. In fact, I've worked out that, if I'm really dedicated, spend lots of time reading, don't read anything I've already read, and don't buy or borrow any more books at all, I could probably get through my "unread" books in, oh, about nine years.
Of course, I'm not about to stop rereading books, or buying new ones. So it's going to take a little longer than that.
I've always preferred buying books to borrowing them, because it gives me a greater sense of connection to the story: If it's my book, then in a sense it's my story. What I'm recognizing now, though, is that life is short (and some of these books are quite long), and it's simply not going to be possible for me to reread all, or even most, of the books I currently own. On the other hand, there are some that I know I'm going to want to read again and again (in An Experiment in Criticism, C. S. Lewis mentions that book lovers will reread their favorites ten, twenty, or more times during the course of their lives).
I'm beginning, therefore, to see a need to distill my collection, and particularly to decide right after reading a book, while it's still fresh in my mind, how likely I am to reread it, and to sell or donate the ones that seem reasonably unlikely. The thing I hate about this plan is that it will inevitably involve getting rid of a lot of good books. I've already decided to let one humongous fantasy series go simply because it's so incredibly long that it would take me two or three months to get through it again.
Naturally, I'll also be buying fewer books. I'd say I'll be visiting the library a lot more, but there's still the matter of all these books I already own but haven't had time to read yet. (This is what I get for going to library sales on $2-a-bag day.)