I wish there was an option to sync all music EXCEPT what is in a certain playlist.
Piece o' cake...
Create a regular playlist and name it something like Not on iPhone.
Add songs to it you do not want to sync.
Create a smart playlist named something like Sync to iPhone.
Match all rules;
Playlist is Music
Playlist is not Not on iPhone.
Select the iPhone in iTunes and click the Music tab.
Tick Sync music and tick Sync selected playlists, Artists Albums and Genre.
Tick the Sync to iPhone playlist and click OK.
If you want to change what is on the iPhone, simply add/remove items from the Not on iPhone playlist then plug in the iPhone.