With fewer flights and more people flying, it's no wonder more travelers with confirmed reservations are being "bumped" (denied boarding) than ever.

Through August, the Air Transport Association reports that airlines filled 81.1 percent of their seats on domestic flights versus 77.0 in the same period in 2005.

The result is that involuntary bumpings rose from 25,041 in the first six months of 2005 to 33,513 in the first six months of this year, according to the U.S. Department of Transportation (DOT).

The best thing any passenger can do to avoid being bumped is to understand the rules of the game. But beyond understanding, passengers need to be proactive and assume that everything that can go wrong will go wrong.

Which means, notes David Stempler, president of the Air Travelers Association (a consumer group), that if you absolutely have to be on a specific flight "you need to protect yourself as much as you can."

- When you make an airline reservation, get a seat assignment.

- If you can, take advantage of electronic check-in on the airline's Web site. You can check in and print out a boarding pass 24 hours prior to departure.

- If you're checking in at the airport, get there early to get your boarding pass and be at the departure gate at least an hour before your flight is scheduled to leave, even if the airline says 45 minutes is fine.

Airlines traditionally overbook flights to compensate for no-shows--passengers who don't cancel a reservation or miss a flight. As infuriating as the bumping experience may be, the carrier's action is legal.

There are two types of bumping--voluntary and involuntary. Typically, when a flight is overbooked, the carrier will ask for volunteers--people who don't have to reach their destination at a particular time.

When a carrier seeks volunteers to give up their seats, the DOT requires that passengers be offered compensation, but doesn't specify how much.

So the airlines can negotiate with passengers based on guidelines given to gate and ticket agents. For example, Stempler said, an airline might offer volunteers a $100 or $200 or $300 credit, but he suggests passengers ask for a "free round-trip ticket to any place on the airline's system." However, he said, since some volunteering passengers might accept a lesser credit, be prepared to settle for less.

What airlines offer volunteers is up to the individual carrier. At United Airlines, for example, a spokeswoman explained: "We compensate a customer who volunteers a seat with a free ticket voucher good for one round-trip economy class ticket within the 48 contiguous United States. We also provide travel credits if a customer would rather have credit toward a future ticket instead of a free ticket." Travel credits, she said, range from $200 to $600, depending on the delay.

A US Airways spokesman said its policy is to offer a voucher for future travel, almost always a meal voucher and a hotel voucher if the next available flight is the following day.

A colleague and his wife flying from Chicago to Rome opted to volunteer for a later flight when their original American Airlines flight was overbooked. For their inconvenience, they each received $500 flight vouchers good for a year, but in the process the carrier lost the wife's luggage for three days.

In general, airlines say they handle compensation on a case-by-case basis.

But before you settle on any denied-boarding compensation, the DOT's "Fly-Rights" guide advises you to ask:

- When is the next acceptable flight on which the airline can confirm a seat? If you are offered standby on another flight that's full, you could be stranded.

- Will the airline provide other amenities such as free meals, a hotel room (if an overnight stay is required), a phone card and ground transportation? You don't want to spend your own money to accommodate the airline.

- And what happens--see above!--to checked luggage?

Under the DOT's involuntary bumping rules, airlines must give passengers a written statement describing their rights and explaining how the carrier decides who gets a seat on an oversold flight and who doesn't. Among the criteria used by carriers to determine who gets bumped: time of check-in (last check-ins are prime candidates), price of ticket and frequent-flier status.

Here's the denied boarding compensation prescribed:

- If you are bumped involuntarily and the airline arranges substitute transportation that is scheduled to get you to your final destination (including later connections) within one hour of your original scheduled arrival time, there is no compensation.

- If the airline's substitute flight is scheduled to arrive at your destination between one and two hours after your original arrival time (between one and four hours on international flights), the carrier must pay you an amount equal to your one-way fare to your final destination, with a $200 maximum.

- If the substitute flight is scheduled to arrive at your destination more than two hours later (four hours internationally, or if the airline doesn't make any substitute travel arrangements for you), the compensation doubles to 200 percent of your fare, or a maximum of $400.

- You always get to keep your original ticket and use it on another flight within a year. The denied-boarding compensation is essentially payment for your inconvenience.

As the U.S. has its denied-boarding rules, so does the European Union for flights that depart from an EC country (DOT rules apply at U.S. airports).

For a voluntary denied-boarding in Europe, the airline must provide the choice of either a refund of your ticket (with a free flight back to initial point of departure, when relevant) or alternative transport to your final destination.

If you are not a volunteer, the airline must pay you (dollar conversion based on euro rate of $1.25435):

- 250 euros ($314) for flights of 932 miles or less.

- 400 euros ($502) for flights between 932 and 2,174 miles.

- 600 euros ($753) for flights of more than 2,174 miles.

The EU also says that airlines must give bumped passengers meals, refreshments, hotel accommodations when necessary (including transfers) and phone calls.

As in the U.S., foreign carriers apply their own rules to determine who gets bumped.