Merlin Mann recently wrote two posts about managing your music library using iTunes Smart Playlists. His suggestions for making music-only playlists (for those that have a lot of podcasts & audiobooks in their libraries) and the "sure you really like that?" playlist are especially helpful. One of my recent favorite Smart Playlists is helpful for discovering good stuff that I haven't listened to in awhile:
The Last Skipped bit is in there because while listening to this playlist, I found myself skipping stuff I didn't want to hear and that rule gets it out there so that it doesn't come up again. An item on my Smart Playlist wishlist is the ability to measure popularity acceleration (basically, something like "gimme the most played over the last week"), but there's no way (that I can find) to ask iTunes how many times a song has been played in the last x days.
Several more Smart Playlist suggestions are available at smartplaylists.com and Andy Budd.