iPod users?

Ok... well I've had my iPod for about 4 years now, and have never really used the syncing feature (call me a control freak or whatever, I just like to control what gets transferred and when).

Now, in an effort to keep things more organized, I've started Syncing. However... when I add a few new songs to the iTunes Library, and click "Sync", it completely erases everything on the iPod and systematically copies it all back onto it. I don't understand why it's doing this at all as it's already done this once and is registered to this copy of iTunes.

There's an option to only copy certain playlists, but this does no good it just erases everything else and re-copies the selected playlists onto the device. Am I doin something wrong here?

Thanks guys