It's your Donald Duck...
Helps to be a member of the frequent flyer club.
The will always upgrade the highest levels of club membership first.
I used to fly a lot, long haul every other week. When I got to Platinum with KLM the upgrades came pretty often.
All the other advice about being polite and smart is bang on. However you could be the nicest smartest guy in the world, but if the guy in front has a gold card AND even if his A*** is hanging out of his trousers, he will get the upgrade and you won't...
Even economy has different ticket types. Fully flex, changeable for a fee, fixed use or lose... And they will upgrade the most expensive first. So if you paid £700 for the economy seat next to me when I paid £300 for it, you'll get the upgrade not me...
An upgrade is nice, but you can get some right stuck up tools in there. The sort of folk who treat folk on less than 400k a year like dirt. Sure you don't have to talk to them, but I get ratty even sitting close to them...