God, I could discuss iTunes metadata and smart playlists for hours! No, really. I use a lot of what other people have mentioned, though I'm not so into ratings. I might make a note to delete something that I really hate (on the Hipster PDA perhaps?) and then delete it later, but I also find that some music grows on me and I like to keep it around. Some things I do that haven't been mentioned yet are:

Playlists based on when items were added to the library, so I can hear stuff that is "new to me".

Playlists based on how recently I've played something. This keeps everything fresh and insures that I never get that feeling that my "random isn't really random" (a false but common perception) because songs are removed from the playlist as soon as I play them and sent to the bottom of the pile. I have one playlist that's my 100 least recently played songs, if I really want to go back, but then I have a music-only (also minus holiday music) playlist that's limited to the 20gb of least recently played stuff out of the 70gb of mp3s that I have on my machine.

The "comments" field is nice, too. I added a custom tag to all of the music that my wife doesn't hate, which is really helpful. Also, I have holiday music that is classical, hip-hop, rock, jazz, etc. So, instead of using "holiday" or "Christmas" as a genre, I put "xmas" in the comments field. I wish someday Apple would add a way to use a controlled list of tags to append to a file, kind of like how "genre" works, with an auto-complete feature, except that you could add more than one to a track.

I have a "podcast" playlist that is set up to compile all of my podcasts that aren't from KEXP (the world's best radio station and the only music podcasts I currently subscribe to) where the play count is less than one. This allows me to just hit play and listen to one podcast after another rather than selecting them individually. (My podcast subscriptions are also set to automatically update every hour and delete themselves when the play count is greater than zero, so I never have a backlog of out-of-date 'casts hogging up my disc space.)

Most importantly, and here's where the fact that I'm a librarian/metadata specialist by profession really becomes obvious, I strictly control all the metadata in my library. Whenever I add new stuff to the library, it goes into a special "dumb" playlist and waits for metadata additions and corrections. Most importantly, people use the stupidest genres to tag things. For god's sake, what use is it to call something "unclassifiable"? Genre headings should, of course, be based on the particular needs of your collection. If 1% of your music is classical, then it's probably not important to worry about subdividing it into "baroque" etc. When a genre gets to big, think about subdividing it, though. I created my own genres that work like this "Rock.Punk.Hardcore" or "Reggae.Dub" so that I can play a fairly specific genre or use a smart playlist to select songs where the genre "contains" (as opposed to "is") reggae. That way I can easily pull together the larger "umbrella" genres. Geek on!