i didnt check if you've cross-posted, so i'll tell you anyway.
create a new playlist, drag and drop ALL the music that you want on your ipod into that playlist.
connect your ipod. when it's connected and you get your "there's not enough space on here" popup, go into ipod preferences (edit>prefernces>ipod tab) and there's some update options in there. there's automatic update, update from the following playlists and manually update. if you want to manually drag and drop songs on there, check that...if you want to use your playlist, check "update from playlist" then select the playlist you just created.
as it'll tell you, if you choose to update manually, you have to eject the ipod manually too.
hope that helps