When I moved a couple of years ago, I got rid of over half the books I'd accumulated in my old residence. There were about 5000 books in the house and I ended up keeping right at 1600. I donated the majority of the books to the local library system for their annual book sale. The running gag that year was that I WAS the library sale.
Anyway, having escaped the clutter and having decided I would never again allow that much stuff to stack up, I have adopted a new policy at my new dwelling. I will buy no more bookshelves. So if my exisiting shelves get full, then stuff has to go. It's not as hard as it sounds. Once I got rid of so much of my material possessions during the move, I found that it was much easier to let stuff go. Things that I had thought I'd keep forever, I've ended up parting with, and I haven't missed them.
Tonight I filled a copier paper box with paperbacks. That makes three such boxes I've filled over the last six months or so. These will eventually make their way to next years library sale. I've also started looking at some of the books I've collected but no longer read or refer to. Basically, if I haven't touched it since I moved, I probably don't need it. (This goes more for fiction than for my reference books.)
Other than my new more 'Zen' attitude, the other thing that makes it easier to get rid of old books is the Internet. There are so many on-line book sources that there are very few books that I couldn't get again in a couple of days if I decided I really wanted to. Books that I once had to search for by haunting used bookstores, sending out want-lists, and ordering print catalogs, can now be acquired with a few keystrokes. So I don't have to horde hard to find books just on the off chance that I might want to read them again some day.
There are exceptions of course. I have some truly rare books and also some volumes that have sentimental value. I'm not likely to ever get rid of those. But pretty much anything else always stands the danger of being culled.