At one time, "schmoozing" commonly worked if one knew what to say. But nowadays, with elites upgrading, others on award tickets and of course "full fare" F passengers, these small cabins fill up quickly.

I'll share one trick though that once worked for me. I was flying an airline for the first time and had just signed up for their FF program. Upon checking in, I asked if there was an inexpensive way for me to upgrade to F. (Of course, I knew there wasn't.) In so many words, the agent said "no." She proceeded to print out my boarding pass and the machine jammed. She kept apologizing for making me stand there while she tried to fix the machine. Every time she apologized, I just smiled and said "no problem." Finally, when she did print out my boarding pass, it was for a seat in First Class.

The point is that it pays to be friendly and it also pays to let them know you're interested in upgrading. If the cabin isn't full and the agent has the authority, you may occasionally get lucky. I think the gate agent is your best bet. Others, more knowledgable than me may be able to elaborate. Good luck!!