Quoted: To share music back and forth between your iPod and a computer (Mac or PC) you need to set your iPod to update manually rather than to sync automatically with iTunes. Manual updating allows you to delete songs in iTunes while keeping them on your iPod, or to keep songs in iTunes and remove them from your iPod.

I wanted to dot this again and emphasize what I found here that was so important... I wanted to record to iTunes using Apple Lossless (for quality, and since my SqueezeBox will stream lossless) but when filling an iPod, I want to use MP3 (for size). What I did not realize was that you can bring into iTunes as lossless, then convert 'in-place' to MP3 (which does not replace but places side-by-side). Then you can resort on 'Kind' to separate lossless and MP3 and then throw all your MP3 files onto the iPod. Sweet.

"Start by ripping your CDs into Apple Lossless format. When you want to move some selected tunes to your iPod or other MP3 player, set your Importing preferences to the correct format and choose Convert Selection from the Advanced menu. Put your iPod in manual-update mode to avoid also transferring the Apple Lossless versions. Then drag the newly converted files onto the iPod icon."

Quoted: Start by ripping your CDs into Apple Lossless format. When you want to move some selected tunes to your iPod or other MP3 player, set ...

Ken Rockwell on itunes and recording quality.