How would you like to fly round-trip to London for $200? How would you like it if you had only 36 hours to decide whether to buy?

American Airlines, a unit of

AMR

(AMR)

, last week announced this exact sale between London's Heathrow Airport and 16 different airports, including ones in Boston, New York and Chicago. The price ran as low as $200 round-trip from New York, which, excluding tax, was just as cheap as a round-trip from New York to Boston. (With tax, tickets would have been $300 -- about the cost of a round-trip flight to Las Vegas.) The sale ran for a day and a half.

By forcing travelers to act fast, American hoped to spur last-minute travel along trans-Atlantic routes that are less popular in the winter. Other airlines are adopting "spot sale" tactics as well, with

ATA

(ATAH)

offering a one-day sale on flights to Hawaii, and

Northwest Airlines

(NWAC)

kicking off a two-week holiday fare sale last week.

Luckily, Internet travel sites like Orbitz, Travelocity and Expedia offer tools that can alert customers whenever a market goes on sale.

"

The airlines are doing the sale to ... get people to make an impulse decision to buy. They're not looking to sell seats over a long period of time," said Geoff Silvers, director of e-marketing for Orbitz.com. "In order to provide service to London, they have to use larger planes. They have no choice on trans-Atlantic flights. That sets them up with a large number of seats they can't control. The flights to London Heathrow don't allow for them to simply stop flying in the middle of the winter."

While American forces customers to act fast to get a deal, it doesn't force travelers to travel right away, which is why people who missed out may be kicking themselves. In fact, the London sale was good for travel from Nov. 12 to March 30. American's sale even covers April and May as well, although prices tick up to $300 round-trip (without tax), since demand for London increases in early spring.

So how can you cash in on these sales? There are a number of ways. Travelocity offers the Fare Watcher, to help customers track sales on their favorite routes. After creating a profile with Travelocity, travelers can enter up to five trips they'd like to take, along with some price variables, and the service will automatically alert them when fare levels change.

"What you do is set the city you're looking for and the destinations you want to head to. Fare Watcher will watch the fare and alert you any time the fare either drops by certain amount or comes down to a certain range," said Amy Ziff, author of the "Ask Amy" travel advice column for Travelocity. "It will tell you about trips to London every time it drops $50 or more. Or you can say, 'Should it drop below $200, then tell me.' You don't have to do anything; it will do it instantly."

Orbitz.com offers a similar feature. Travelers can download the Deal Detector, which is a program that continuously scours Orbitz's database of flights in search of cheap flights. As with Travelocity's Fare Watcher option, all travelers have to do is enter their favorite routes with prices they want to pay. When Deal Detector finds fares that match the criteria, users are automatically emailed.

A number of other travel sites, like Expedia.com, allow users to set up profiles that name a favorite airport so they can get a weekly email detailing fare changes and sales.

While these Internet tools make it easier to keep abreast of the latest sales, travelers who miss out shouldn't fret too much. Typically, other carriers will move in to match sales offered by rivals and take them a step further, adding more cities, lower prices and longer time periods for travel. In fact, American's sale to London was a response to a similar sale offered by

British Airways

.

"If a carrier has a one-day sale and it's over before another carrier has matched, be assured the rival has a one-day sale planned so they will not miss out on an opportunity," said Harrell. "And often, one-day sales are matched by 10-day sales."

Indeed, according to data from Bob Harrell, principal of Harrell Associates, a New York-based airline consultancy firm, a number of routes have been put on sale at a time when leisure fares, overall, have been rising. (The average national leisure one-way fare rose 5% last week and 10% the week before that, but a combination of competition and slumping demand has reduced fares in select markets, especially on long transcontinental flights.)

While Harrell noted that the fare comparisons can be "horrendously volatile," he added that "it's quite possible that we'll see more sales before the end of 2003."