An upgrade is like winning a lottery in the air travel world. No one has a full-proof way of getting an upgrade every time, but here's some insight into how they are chosen. Sometimes flight attendants need to move people around for weight and balance purposes. This means moving passengers from the back of the plane to the front cabin classes to redistribute weight. So the trick is always to book your seats in the back of the plane!
Now, if they need to move people to first class, you will be the first to know!