You can also look around the ticketing counter and gate of your flight and then volunteer your upgrades for them if space is available. Or, many airlines allow you to swap your F for their Y seat. Here are a few threads on this topic:
http://www.flyertalk.com/forum/showthread.php?t=333622
http://www.flyertalk.com/forum/showthread.php?t=337738
http://www.flyertalk.com/forum/showthread.php?t=338992