How are you adding music? If you want it to update automatically, then make a playlist in iTunes, add all the music you want on your iPod to to this playlist, then in the preferences, select "Update selected playlists only" and select the playlist you just made. Now everytime you plug in it will compare that playlist to whatever is on your iPod and update any differences. In future you can add or delete songs from this playlist and it will automatically do the same to your iPod when you plug in.
Another way to do it is change to manual update in the preferences. Now all you do is drag and drop the songs that you want from your library onto the iPod icon in the source window.