Join the airline's frequent flier program and fly enough times per year. Many will automatically put you in the queue for a free upgrade if you fly enough. It also helps if you are presentable (e.g., wear a nice suit) and polite. So few people who travel are polite these days, that smiling and not being rude will make you stand out.

Once, on a flight to Japan, I folded a piece of origami while waiting in line to check in for the flight. I finished the origami by the time I reached the front of the line, set it on the counter, and the check-in agent asked if I'd like a complimentary upgrade. I said yes, then asked if my boss could be upgraded too (he had checked in ahead of me). She gave him a free upgrade as well. Mind you, this wasn't the simplistic cranes that most people know how to fold, but a dragon involving more than 100 folds.

If your flight is overbooked and you offer to fly in a couple hours, you can ask the agent to put you in first class if there is a seat available. Worked many times. The airlines are willing to do this because they overbooked the flight.

You have to fly A LOT. When I used to travel for business weekly, it was not uncommon for me to get upgraded. Now that I'm exclusively a pleasure traveler, I rarely get upgraded.