The other way to certain only the specific songs you want is to create a new playlist that contains all the music you want to sync.
When you've done this and are ready to sync the playlist, begin by syncing your iPhone or iPod to your computer. When the sync is complete, click the Music tab on the management screen in iTunes.
Specify Playlists to Sync
In that tab, choose the selected playlists, artists, albums, and genres button and then choose the playlist or playlists you've just created and nothing else in the other boxes. This will ensure that only the new playlist you've created will be synced to your iPod or iPhone.
It's important to understand that if you only choose your new playlist(s), everything else will be removed from your iPod and just the contents of the playlist(s) will be synced. If you want to sync these playlist(s) in addition to other music, check items in the Artist, Genre, and Album boxes for the other items you want to sync.
When you've got everything set how you want it, click the Apply button at the bottom right and you'll be all set. Resync your iPod or iPhone and all the changes will take effect.
Maintaining the Playlist
After you do this, you must remember that only music added to your new playlist will be transferred to your iPod or iPhone; additions don't happen automatically anymore. This means that whenever you add a new CD to your library or buy from the iTunes Store, you have to manually add those songs to your new playlist (unless you use smart playlists).