Smart Playlists are one of the coolest yet most underused features of iTunes. Part of the reason might be because people don't know what to have their Smart Playlist do.
I've mentioned Smart Playlists before, including a link to how you can set up a live music Smart Playlist. Here's another idea:
I like to have a playlist of similar bands, such as one with the Beatles and artists that are influenced by them (or sample their music). To do this I use put keywords in the comments field.
For artists in my iTunes library that I want to include in my new Smart Playlist, I select their tracks (using multiple selection to make things go faster) and select File > Get Info. Then, in the Comments field, I type the keyword "beatles".
Next I select File > New Smart Playlist. There I set it to match if Artist or Comment contains "Beatles", matching any of the conditions and keeping Live Updating checked (see screen below).
Now any artist that matches -- for me, that includes the Beatles, Elliot Smith, DJ Dangermouse's "Grey Album", Blur, and others -- will appear in my playlist. And since live updating was checked, any new track with "beatles" in the comments field will automatically be added to the playlist.
You don't have to just put band names in the comments field -- create your own Smart Playlists using descriptions ("angry," "party"), subgenres ("surf," "classicrock"), or anything you want. And you can put multiple keywords comments if you want a song to appear in more than one playlist (for example, "ramones 1970s fast").