While connecting the iPod to the computer, hold down the Shift + Ctrl keys in Windows (CMD + Option keys on a Mac). This will stop the iPod from auto sync’ing with iTunes and the iPod will appear in the source list. Wait until you are sure the iPod has mounted, and that it will not auto sync and then you can let the keys go. This may take between 3 and 25 seconds depending on your computer.
Then you can right click on the iPod icon in iTunes’ source column and select iPod Options, then check the box that says “Manually manage songs and playlists”. You can now manage your library without losing any songs on the iPod.