Summer is a peak time for "bumping" when airlines have too many passengers for the seats on a flight.

Airlines regularly overbook flights to account for no-shows, and volunteering to be bumped can be lucrative. For waiting a few extra hours at the airport, you receive travel vouchers that could be worth enough for a free round trip.

But before you volunteer, know what you're getting. If you're told you'll be put on the next available flight, ask when it will be. Also, make sure you will have a confirmed seat.

If you agree to be bumped, tell the agent that if your seat isn't used, you want your original assigned seat or a better seat. If you don't specify this, you could end up in the back of the plane.

You'll want vouchers in a dollar amount, instead of a free round-trip ticket. The "free ticket" vouchers are often subject to the same capacity controls as frequent-flier tickets, so it may be hard to find a seat. If you volunteer early and are offered $100, then passengers later are offered $200, ask for the additional amount.

Before accepting a voucher, ask about restrictions such as blackout dates and expiration dates, and make sure it can be used toward the cheapest fares. Ask if the voucher can be redeemed online. If not, you may end up losing $10 to $15 of the value, since most airlines charge for dealing with a live agent.

If you're delayed for more than a couple of hours, get the voucher plus a phone card, meal voucher and airport club pass. If you're delayed overnight, ask for a hotel voucher. You may be refused, but it doesn't hurt to ask.

If you want to increase your chances of getting bumped, book flights for Friday and Sunday afternoons or early Mondays. Avoid the last flight of the day. Airlines are less likely to overbook these.

It's best to travel with just carry-on luggage when you're trying to get bumped. That way, your baggage won't be sent on your original flight without you.

If you want avoid being bumped, check in early. Often, the last people who check in are the first to be involuntarily denied boarding.