Join the frequent flyer club of the airline with whom you are flying. Being a frequent flyer will allow you to obtain mileage credit for the flight you're about to take and, if you're a frequent flyer, it increases your chances of an upgrade. Passengers whose loyalty has boosted them to elite status with the airline are the first passengers considered in the long queue of upgrade-seekers.
Book your flight on a wide-bodied plane, like a Boeing 747 or 777 or an Airbus A340 or A380. Wide-bodied aircraft like these have more business and first class seats, which increases your chances of filling an empty business class seat if coach is oversold.
Fly from a smaller hub that is off-the-beaten path for your airline. If you fly from an airline's main hub, you will probably be up against the most seasoned frequent travelers when requesting an upgrade.
Check in early and ask to be considered for an upgrade. In fact, if you can check in online 24 hours prior to your flight, do that instead of arriving at the airport early to check in. The sooner you put yourself on the list of confirmed passengers, the better a chance you will have for an upgrade if it becomes available at the gate.
