You can easily control what gets synced onto your iPod (or iPhone) by using playlists or excluding specific tracks, but the former isn't comprehensive and the latter stops you playing the full contents of your iTunes library on your computer. Blogger Scott Buckley details a clever use of playlists that lets you blacklist specific tracks and sync everything else to your iPod.
As Scott points out, if you exclude tracks from the sync list for your iDevice, they won't play when you're using iTunes as a desktop music player. His solution? Create a playlist of the songs you don't want to exclude, and set up a rule that tells your iPod to only sync tracks that are not on that list. That makes the list easy to edit and ensures that your desktop playback experience isn't affected.
Hit Scott's post for the full details. If you're not up to speed with building an iTunes smart playlist, check out our Lifehacker 101 guide.
iTunes: Sync Blacklist Without Unchecking [Buck Etc]