If your music library is bigger than your iPod, you probably use playlists or iTunes' checkboxes to tell iTunes which songs you do and don't want synced. Mac|Life recommends using the star system instead.
Everyone has slightly different needs when it comes to music libraries and syncing, but this method might work for some of you. I myself have found that I didn't like iTunes' checkboxes, since it does more than just exclude them from my iPod — it also skips them when I'm playing through my library, which I don't want. You could manually create a playlist of tracks you want to incldue or exclude, but that isn't the most user-friendly. You can't see, from your library, if a track is already on the list, and if you want to quickly mark something as "sync" or "don't sync", you have to see if it's on the playlist, then drag it on or off.
Instead of dealing with these imperfect systems, Mac|Life recommends just rating unwanted songs as one star, and creating a smart playlist containing all tracks where the Rating is not one star. Then, just sync that playlist to your iDevice. You'll not only be able to view the playlist as-is whenever you want, but you'll also be able to see, when you're scanning through your library, which songs are and aren't part of your iPod's library. Plus, iTunes won't skip them when you play them on your computer (seriously, that feature is really annoying. If I didn't want it to play I wouldn't have it).
Of course, this doesn't necessarily work as well if you actually use the star rating feature. It might, if the songs you aren't syncing are the lower-rated songs, which is likely — but if the two don't coincide this method won't work. For others, though, this is one of the better workarounds I've seen for managing a limited version of my library without going a little crazy.