In 36 hours, you need to be in Chicago and you don't have a plane ticket yet.

Maybe it's an unexpected business trip or a family emergency -- or you simply crave authentic Deep Dish Pizza -- but when you need to fly at the last minute, using the Internet to book your flight can make things a lot easier and cheaper. But with dozens and dozens of online travel sites out there, ensuring that you get the best last-minute deal on a domestic flight can be a time-consuming and complicated process.

Last year, more than 26 million people bought airline tickets online -- more than 15% of airline revenues come from the Web, according to Forrester Research. And while the Web may make shopping for airfares a snap, there's no one-stop shop around that can guarantee the lowest prices. And so online travel shoppers scour the Web, visiting an average of 3.5 Web sites before they purchase their tickets.

Here are the steps you can take to get the best deal in a hurry.

Step One: Take the Broad View

In order to keep supply in line with reduced demand, airlines rely on yield management, constantly adjusting their fares on a moment-to-moment basis. Because of this, a round-trip flight between Miami and Dallas can cost $500 one day and $300 the next. The first thing you need to do is get a ballpark figure for how much that fare should cost.

The best way to get the big-picture view on fares is by checking one of the big three online travel companies,

Expedia

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,

Travelocity

(TVLY)

or

Orbitz

. For the most part, the flights listed on all three sites are largely the same, culled from one of the four global distribution systems, or GDS, where the airlines publish their fares.

"We have contracts with 40-plus airlines that are charter associates. They give us all the fares that they publicly load into the GDS, plus the fares they put up on their own Web sites, so basically, we get anything that is publicly available," said Con Hitchcock, chairman of Orbitz's consumer-advisory board. "What we don't get are things like affinity fares and other corporate discounts."

As far as search engines go, each has strengths and weaknesses. But Orbitz is the best at showing you everything on one screen at the same time, which is good when you're looking at a busy route, like New York to Chicago. But it makes sense to check all three because you never know when Travelocity will have a flight time the others are missing, or a cheaper fare in their system.

Step Two: Know Your Airlines

Chances are after checking out the big three, you'll have a very good idea of what's out there, but don't reach for your wallet yet. While the big three receive a cut of every ticket they sell from the airline carrier, they also add in a service fee a la Ticketmaster. At Orbitz, the fee is $10 for a domestic flight; at the other two, it's $5.

And while the big three carry a large selection of fares, they're not necessarily representative of everything that's available on the Internet, especially when it comes to international flights. For starters, not every airline allows them to sell their fares, with low-cost carriers

Southwest

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and

JetBlue

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two of the more notable abstentions. Once you're done checking the big sites, it's time to check the airline Web sites to see what they have.

"If it's out there and it's a publicly available fare, then we'll list it," said Hitchcock. "But if you go to individual airlines, there may be a little bit of a difference. We are addressing that by setting up a system called direct connect, to go to individual airline systems and bypass the GDS."

But that hasn't happened yet. By shopping the individual airline Web sites, not only will you be able to see flights you otherwise would have missed, you can also skip out on the $10 or $5 fee the online guys are charging you. Most airlines don't charge a fee when you buy directly through them and some even have cool features that allow you to pick your own seat.

Your best bet, especially in a last-minute situation, is to write down every carrier that serves the route you're on and visit each of them. Don't forget to also check Southwest and JetBlue.

Step Three: What You Don't Know Can Be Cheaper

By the time you get to the end of the second step, you're probably ready to buy that plane ticket. But last-minute travelers who have some flexibility on departure times would be wise to check deeply discounted fares.

When an airline can't sell its seats at the published price available in the GDS or on its own site, they take this distressed inventory and sell it to sites like

Priceline

(PCLN)

, Hotwire or BestFares, who buy fares for pennies on the dollar and resell it to you on the cheap.

But because airlines don't want rivals and customers knowing they're dumping seats into the secondary market, a site like Priceline won't tell you which carrier you'll be flying and what time until

after

you've bought the ticket. This is why they're called "opaque fares," because the time and carrier aren't transparent.

"Today, the airlines sure are calling a lot of things distressed merchandise," said Tom Parsons, CEO of BestFares.com. "Even to this day, some airlines don't want the public to know what we're selling."

For $59.95 a year, BestFares members can see the flight times and carriers before they buy, making them a Sam's Club for airfares. But even at sites like Hotwire and Priceline, which are more opaque, as long as you're flexible when it comes to time and carrier, the deals are staggering -- and totally exclusive to the place you're buying them.

Right now, you'll find some amazing deals on Caribbean travel in the opaque market. Let's just say you wanted to go from Miami to Cancun on Thursday, March 6, and come back on Sunday, March 9, leaving in the afternoon both legs of the trip. Travelocity's best fare is a $317 roundtrip flight on American Airlines, Expedia's best is a $357 roundtrip flight from Aero Mexico, while Orbitz has a $278 from the same carrier.

"Looking at the GDS, I see Miami to Cancun roundtrip for $360 or so," said Parsons. "We have it at BestFares for $179."