There are various ways to be upgraded and different airlines follows different systems. One answer doesn’t fit all the systems.
A. Paying outright
Basically, when you are checking in , ask about if there are any upsell offers. Mostly they do. They would charge you certain fee that is just a small percentage of revenue required.
Many international airlines offer to bid at the gate/check-in counter. Someone with the highest bid is offered the seat up ahead.
If you are a customer for the airline and have accumulated miles through different hacks. (Something that I cover extensively on my blog). You can pay outright amount of miles required to be upgraded. Policies regarding miles upgrade is changing.
When airlines have oversold the flight. They would bump certain passengers up to business class or even first class. How does that happen? How to maximize that?
1. A person with the highest FF status.
2. A Person with highest booking class
3. A person checking first/last at the boarding gate depending on airline system.
If you are an influencer in any way and airline Personel decide that they would like to get into your good books. They might bump you up.
Frequent flyer benefits
Many airlines offer different benefits to their frequent flyers. Some outright offer unlimited upgrades which technically entitles you to be upgraded on every flight as long as you clear for the upgrade. Other airlines offer a certain amount of upgrades for their frequent flyer members.
IT’s YOUR LUCKY DAY
I would create a separate post for the art of getting bumped up.