Welcome to FT!
Suppose someone posted in an automobile forum:
Does anyone here have suggestions on how to get a car, assuming that the person seeking the car has bad credit, has no trade-in available, doesn't want to pay for it, etc., etc.? In other words, are there things to say or do that would convince a car salesperson to give you a car?
The upgrade situation is similar. Competition has increased the number of people legitimately entitled to upgrades. If more are needed, those with some claim to priority (e.g., elite status) normally exceed the number of upgrade seats available. Airline employees are under increasing pressure to follow the book and not grant personal favors.
This is not to say it never happens. If you can become a famous, easily recognized movie star or athlete before your next flight, that would help.
Otherwise: your chances are slim to none, and Slim just left town.