Clinton's post won't help you, as the iPhone backup is not stored in your iTunes library. Follow these directions:
1) Make sure that iTunes is closed.
2) Move the folder called ~/Library/Application Support/MobileSync/Backup/ to the drive you want to back up to. For instance, an external drive.
3) Launch Terminal Applications/Utilities/Terminal and type in the following command line:
ln -s /Volumes/DriveName/Backup/ ~/Library/Application\ Support/MobileSync/Backup