Chances are most of you don't actively rate every song in your library, but you probably do skip tracks you don't like as much. If you'd rather not sync those "disliked" songs to your iPod, iTunes has a feature that lets you ignore them.
Basically, if you skip a song more than a few times, iTunes will decide you don't like it and banish it from a specific smart playlist for a month. When you're creating a smart playlist, choose "Last Skipped" from the drop-down menu and set it to a specific period of time (say, a month). Now, that playlist will ignore any songs you've skipped in the past month. I'd also add another filter for the number of skips, as shown above. That way, if you skip a song once or twice, it'll still be synced to the playlist. After all, you don't want iTunes to think you hate a song just because you skipped it once.
It's a little feature, but much less work than actively rating all your songs and filtering one-starred songs out of your playlist that way. And, it's pretty nice if you don't have a ton of space on your iPod and want to keep some of your more mediocre songs from filling it up. Hit the link to read more.
The Point of iTunes' Skip Feature [Macworld]