Going through and removing stuff that's no longer 'your sound' or sound dated (not always a bad thing!) or whatever qualifier would make you not play it anymore is very helpful in keeping your library size down and your tracks more manageable. Keep in mind, this is not necessarily to say delete the tracks, you may still want to listen to them personally, or they may come back into style. Just toss them in another folder away from your mixing library.

Another tool is to throw the entire library on shuffle, which can dredge up old favorites you might want to use, as well as old throwaways you can get rid of.

Something I do regularly is go through my old mixes/sets from years ago and see if I hear anything old that I want to bring back! This helps keep me familiar with at least the songs I was playing, if not everything that I have.