Sometimes, getting a free upgrade means taking another flight.
Voluntary bumping occurs when you agree to give up a seat on an overbooked plane in exchange for some compensation. Most of the time, that compensation comes in the form of a voucher for free future travel. Personally, that's the way I like to receive a reward for waiting around an airport until the next available flight.
But if you want a free airline upgrade, why not agree to be bumped in exchange for a better seat? If there is a spot available, and the flight isn't a long-haul, the airlines might just agree to your request. After all, filling one first class seat could be cheaper than giving you a free trip.
Just how do you go about being bumped? Simply present yourself to the airline on the day of travel as someone who is willing to give up your seat in exchange for considerations in the event the flight is overbooked. If there is an overbooking (and frequently this does occur because airlines have to avoid empty seats), airline personnel might appreciate your offer -- even if it is motivated by more than kindness.