Taking Stock of Stuff

I have a full bookshelf as it is. With school now, I am going to be adding another shelf's worth of material. I need to make room for some more books. Saying something like that prompts to ask: What do I keep? I don't really want to move with a whole lot of stuff in tow. But what do I throw away? I could move my old high school yearbooks and store them elsewhere, it's not as though I need them as a reference all the time now. The collection of old magazines could also stand to be thinned out a bit. Those two moves alone though probably won't create the amount of space I want. I'm hesitant to get rid of old textbooks, being as I'm going to become a teacher, having relevant books will probably help out quite a bit. In the end, I'm not sure what I'll toss, but it's not an easy decision.