You change the update option of your iPod to "manually manage music and videos". Your iPod will not sync with iTunes in this mode, when you want to add anything to it you connect the iPod and drag and what you want from your iTunes library:
Managing content manually on iPod
iPod 101: Fill 'er Up
Note: When changing from automatic to manual, check the "manually manage music and videos" box in the Summary tab then press the Apply button. "Don't" uncheck Sync Music in the Music tab, it will be unchecked by default when you choose the manual setting, if you uncheck it without setting the iPod to manual first you'll erase all the songs from your iPod