We try to have a planned approach to weeding. We take a different area each month to weed but as with some house keeping tasks other things take priority. I recently weeded the entire fiction collection - adult, young adult and PYP. My criteria - condition of the book, how old it is, how often is has been borrowed and whether it serves a greater purpose staying on the shelf (if we have no other fiction set in Kenya for example) I also order many replacements as we go.
What to do with all the old books? This year our secondary school had a field day in the last week. Each Community and Service group had to come up with a game. Our student librarians came up with this game - balancing a book on your head while navigating a trail. It was surprising popular and a welcome change of pace from the other more rowdy and race type of games.