U.S. airlines forced 36,882 passengers to stay behind because of overbooked flights during the first nine months of 2005, government statistics show. That's less than one in 10,000 passengers, and far fewer than the 460,974 passengers who agreed to give up their seats on overbooked flights.

Forced bumping is rare because airlines are required by law to first seek volunteers. Only when the airline fails to attract enough volunteers can it bump passengers against their will.

Airlines may ask customers while checking in if they have some flexibility to stay behind and wait for another flight.

Unlike passengers who agree to be bumped, passengers who are involuntarily bumped have certain legal rights.

They're eligible to collect a cash payment of as much as $400 in addition to getting an alternative flight. The amount depends on the length of delay. Fliers who reach their final destination within an hour of initial arrival time aren't entitled to anything.

If you volunteer, be aware of what you can reasonably ask for. Some passengers negotiate better packages by asking, for instance, to waive restrictions on free tickets.

A voucher for future travel is the common currency of voluntary bumping. An airline may also offer meal coupons, a hotel room, ground transportation or free upgrades.

If you want to accept free travel, remember to ask about restrictions such as blackout dates before accepting.

The best way to avoid being bumped is to check in for a flight early, either by using Web check-in or arriving at the airport early. Airlines often bump those who checked in last. Confirm flight time ahead of time, in case of a boarding-time change.

If bumped, ask to be confirmed on the next available flight — instead of flying standby — before clinching the deal.