Ok, I think I've figured out how to move songs into the iPod without having to back it all up (who has the time when they just want to add a new album?). I sympathise with everyone who has lost their music - I recently had a hard drive malfunction, so all the music I have is on my 120GB iPod. Not wanting to have to comb through my external hard drives to re-create my iTunes library just to update a new album, I've been looking for a way to add music to my iPod without having anything taken out against my will. I'll break it down...
1. Plug your iPod in. Most of the times, iPods are set on autosync. However, if it's from another iTunes library, you should get a pop-up warning you about the fact that syncing will clear your iPod. Cancel, and do not sync.
2. On the summary page of your iPod, at the very bottom, there should be a section called "Options", and a place to check a box called "Manually manage music and videos". Check it off. This will enable you to move your music back and forth without iTunes wanting to clear out your iPod for a new library.
3. Remember, do NOT attempt to move to the music tab and sync your music. Instead, click on the little arrow/triangle next to your iPod name (this is on the left sidebar, under "Devices"). You should now see a list of things on your iPod, from Music, Movies, TV Shows, Podcasts, Audiobooks, to your playlists.
4. Now in order to move your music, highlight whatever album or song you want to move from your iTunes Music Library, and move it in to the Music section under Devices > iPod Name > Music. This should move the songs into your general iPod music folder. Click on that folder and search for your music to make sure it's moved in correctly.
5. If you're trying to move a playlist, make sure you're not moving one playlist into another, because then the playlist you are moving will merge with the other. Just place your playlist somewhere around your iPod's name, and it should move in with the other playlists without merging.
7. You're done! Make sure to eject properly, and if your disk use is enabled, make sure that your iPod is no longer mounted in Finder.
So, I think that's about it. I think this will work better with small amounts of music rather than large libraries, but that's just me. Anyhow, I hoped this helped someone, and I hope this step-by-step is easy enough to understand. Good luck everyone!