I got tired of schlepping all my new music onto my iPhone.
Solution? A "Recently Added" smart playlist.
Click the image for a full sized picture
Since I listen to a lot of LOG podcasts I limit the playlist size to 2 GB so I don't blast my poor 8gb iPhone.
Set up one of these, add it to your list of synced playlists, and you'll always have access to your latest podcasts and music with your phone.