You don't always have to copy every song from your Library to your iPod. If you have 20 GB of music on your Mac but only a 15 GB iPod, it won't even be possible. Consequently, you may want to be selective about which playlists you move to your iPod.
When an iPod is connected to your Mac, you'll see an iPod icon at the bottom of the iTunes screen.
Now when you sync your iPod, only the selected playlists will be updated.
Name your playlists with your iPod in mind. This requires you to keep titles short (the iPod displays only 21 characters). Since the iPod alphabetizes titles, you can "trick" it into keeping your favorites at the top by inserting a hyphen () or a period (.) as the first character in the name.