Thanks to the folks at the travel site Globe Hunters for this helpful infographic. I have had little luck getting upgraded until the past few years. I’m not sure what has helped, maybe it’s just sympathy for an old guy traveling economy. Based on recent free upgrades, I would venture a guess that springing for premium economy seats may be the key to more frequent upgrades to business class. I also concur with the suggestions below to check-in early, join the airline’s frequent flyer program, and be extra friendly to the airline agents and desk staff.
Travel Between The Pages 
It is generally harder on intercontinental flights to get an upgrade since there are so much code shares these days any available seats are filled.
On the other hand shorter haul flights could be a better bet. Such as we had recently to Bangkok!
I’ve had some good luck on upgrades this year on Icelandair (btw the US and Iceland and btw AMS and Iceland) and on KLM flights.I must be getting more charming in my dotage.
LOL. Well keep charming them!