There are two words that are reportedly almost always guaranteed to get you a flight upgrade: "revenue management".
Speaking to Bloomberg, author and frequent flyer Tilly Bagshawe says that, while arlines prefer cold hard cash for upgrades, they also want to please frequent flyers and will release spare seats shortly before the flight for those who want to pay with air miles.
The trick, she says, is to get the seats before they are all snapped up.
The way to get it? Ring up the airline and ask: "Have revenue management released any first-class seats for miles upgrades yet?"
"When they say no, ask them to check or just be put through to revenue management so you can ask when they will release some," she told Bloomberg.