I'm running into a problem too. I just don't play as much as I use to, and there is a lot of money tied up in the games. It's even rarer for anyone else in the family to play on their own anymore (although anytime the kids have friends over they play).
I seriously think over the summer I'm going to trim my collection down some and then as I lose interest in a game it'll get replaced with something I've never had before. I think there will be few I keep forever whereas before I always wanted to keep them all forever.
I'm not quite there because I can't decide what I'll sell, but I think having a smaller number will help me enjoy the depth of the games more and give me more room when I'm working on a project or two.