I can't tell you how many times my post on overriding your iPod sync options to manual sync mode has saved me. I'm traveling this week and needed to charge my iPod from my laption, but I don't want to sync it. No problem – you can easily charge your iPod with USB or Firewire without syncing.
To override your iTunes and iPod sync settings and force iTunes to start in manual sync mode, hold down Command and Option (Mac OS X) when you connect your iPod. This will force iTunes to initiate the iPod connection wizard.
You may be prompted to select your iTunes library or create a new one. (Your existing iTunes library is in your 'Music > iTunes' folder, or at least that's where mine is.)
You may be prompted to confirm the name of your iPod. Simply deselect (not checked) the option for "Automatically update" your iPod.
By over-riding the iTunes automatic sync option, you can keep everything that is already on your iPod, manually manage easily songs, movies, and videos, and charge your ipod without fear of losing what's on your iPod.
(Note: This trick is especially handy if you have an external hard drive or network hard drive for your music library, and normally have your ipod set to Automatic sync mode.)
That's it! Charge away! 🙂