iTunes includes two basic synchronisation options: automatic and manual. Automatic syncing means that your iPod immediately begins syncing all media to your computer when you plug it in. Unfortunately, this can delete media off your iPod, because your iPod will synchronise exactly to the media files you have on your computer. So if you've deleted media files from iTunes on your computer, those files will delete off your iPod as well--when it's in automatic sync mode. That's why many iPod users turn on manual mode, because it prevents iTunes from deleting data off of the iPod automatically during a sync.
Using its USB cable, connect your iPod to your computer as if you were going to sync the device with iTunes.
Run iTunes and as you do so, hold down "Shift+CTRL" (Windows) or "Command+Option" (Mac), in order to prevent your iPod from automatically syncing.
Choose your iPod's name in the source panel. Notice that "Summary" view appears in the main panel.
Click the square next to "Manually manage music and videos" to turn on manual mode, and then click the "Apply" button to save the changes. iTunes will no longer automatically sync and delete music off your iPod.
Sync data in manual mode by choosing your desired media tab and manually selecting the items you would like to sync.