Here’s how I upgraded a friend a couple of years ago: I think a lot of you will understand the underlying tactics, so I won’t spell them out..

My friend was going from LHR to BCN on a business trip. Most of the other people on the BA flight were his colleagues. The only available seat he could book for me was in Biz, but he was in back in Economy.
I don’t eat meat, but I was REALLY looking forward to the hot breakfast in Biz, so at Check-in, when we asked if we could be sat together (i.e. me downgrading) I asked if I could still have the meal I’d paid for but back in coach. There is no way BA would do this. I expressed that I hadn’t eaten as I had expected a good meal on board, but resigned graciously to losing my Biz seat and my Biz breakfast.
At the Gate we were called over. I got my original seat back, and as the flight was full, my companion was first choice for an upgrade.
It’s the only time I’ve ever tried to blag an upgrade – but then, I’m always better at doing things for other people than I am for myself.
Not an approach for most FTers as I was gambling with my own seat. But, hey, he DID buy my ticket.