Airlines routinely oversell flights in anticipation of the fact that some passengers inevitably cancel, change their itinerary, or fail to show up. Why not let the seats go empty? Because air travel is a perishable commodity, and that potential revenue is lost forever as soon as the plane backs away from the gate. Ensuring every row is full on every flight helps keep routes profitable and fares affordable. The drawback – for the airlines anyway – is that predictive models can be wrong. Sometimes there are more passengers than seats and federal law requires that the carrier request volunteers to agree to wait for another flight or, in common parlance, take a bump.

Compensation is at the discretion of each carrier, but tends toward the valuable. Just because airlines are trading worthless empty seats for valuable real estate doesn't mean deals are mutually beneficial. Still, getting bumped isn't as easy as raising your hand. Agents have discretion in choosing who will get bumped and look for candidates without checked luggage, companions, or a hard deadline.  

Sometimes the agent won't know until right before the door closes if a volunteer is even necessary. Disappear for a snack, and you could lose your place in line. Stick around, and you might be able to earn yourself a lot of free miles. Passengers who take multiple bumps in a row, each time collecting additional compensation, often end up with free flights to far flung locations. That's the reward for playing the game right.

Here are three rules for travelers looking to get to take a bump and maximize the payout for the (hopefully slight) inconvenience.

1. Be first in line.

If you think the flight might be full, speak to a ticket agent when you check in and ask to be put on the list of volunteers. When the gate agent arrives – usually one hour before departure – confirm your place in line. Remember: Putting yourself on the list is not a commitment. You don't have to agree to anything until you're happy with the new arrangements.

2. Have an alternate itinerary in mind.

The carrier is required to confirm you on a new flight if you take a bump, and this may be difficult if other flights are also full. If you can offer a solution to that problem, you'll be doing everyone a major favor. Look at alternate airports near where you live and even earlier flights you can try to get on standby.

3. Know what kind of compensation you want.

Focus on things the airline can provide, like credit toward a future flight, an upgrade on the new itinerary, or lounge access. If you don't like the agent's first offer, negotiate. Compensation varies widely but is typically a $200 to $400 electronic voucher toward a future reservation.

Even if you don't volunteer, airlines can bump you. This is called "Involuntary Denied Boarding," and the compensation is generous – one reason airlines try to find as many volunteers as possible. The carrier will rebook you on its next available flight or even another carrier if it means an earlier arrival. You will also be entitled to cold, hard cash: up to $650 if your new flight arrives over an hour late, or up to $1300 if your new flight arrives over two hours late.