The rules today allow an airline to change the fares on a flight up to once an hour, says Alexandra Arguelles, director of product management for Amadeus North America in Miami, which builds the technology that operates many of the airlines' and travel sites' booking systems. Fares are in so-called buckets, which are dictated by the airlines' revenue management software, she says.

If you are flying on a week trip between San Francisco and Boston, for example, you could pay anything from $289 to $824 roundtrip if you travel anytime over the next four months, according to an analysis for Reuters by the suburban Los Angeles-based airfare shopping site CheapAir.com. That $289 fare is only available on five of the possible 120 departures days.

On one Delta Air Lines (DAL.N) route between New York and Miami there were 17 different economy class fares, ranging from $109 to $733 one way, the analysis found. Between Denver to Las Vegas, United Airlines UALUN.UL had available 18 different economy class fares, ranging from $63 to $1,165 one way.

"It's like playing the stock market buying an airline ticket," said CheapAir CEO Jeff Klee. There's no way to pinpoint it. There's no one who can tell the best strategy or the best date to buy. It changes so frequently."

On average, the cheapest fare for a flight will be sold about six weeks before departure, according to a study by the Airlines Reporting Commission, which processes transactions for the travel industry. "If you know far in advance you're going to take a trip, check the fares every week, or more often. Wait for the prices to drop," Klee said. "At that point, it's important that you pounce on it when you see a good deal."

Because only so many fares are available in each price category, he points out that buying, say, four seats at a time, could result in a higher fare since the system will default to the lowest price that has four available slots. So, you could get two or three of the seats at a lower fare, but only if you booked them one or two at a time - risking not buying enough tickets to get everyone on the flight. In those situations, you might want to use a traditional travel agent or the dial-in customer service line like the one offered by CheapAir, he suggests.

As a general rule, Klee said Tuesday, Wednesday and Saturday will have lower fares than the other days of the week. And, he added, taking a weekend trip that departs on Saturday and returns on a Monday typically will be less than leaving on Friday and returning on Sunday.

You can get an idea of what you ought to be paying on a given domestic route by checking the U.S. Department of Transportation Consumer Airfare Report. It comes out quarterly, and is several weeks behind, but has the average fare for thousands of segments, along with the average fares for each segment of the largest carrier in the market and the cheapest carrier.

Maximum flexibility, says Montie Brewer, former CEO of Air Canada, will yield rewards for travelers. If you are not bound by specific dates or times of day, he says, you will be able to shop for lower fares. Airlines and travel sites are making it easier for consumers to find lower fares by letting them look at a larger chunk of time when searching. The airlines want a certain number of people to book in advance - knowing that they have got enough paid seats on the plane before pushing up the price.

That does not always mean that booking as early as possible will result in the cheapest ticket - evidenced by the Airlines Reporting Commission study - although it can. "Airlines don't want to sell too many seats too cheaply too quickly," Klee notes.

Some factors can push fares lower than the initial lowest fare. If competition on a route results in one carrier cutting a fare, its rivals will often follow suit - something a consumer who bought long in advance will not benefit from, Arguelles says. And, if the plane is running far below its expected bookings, the airline could push down fares to attract more passengers. Many of these deals are offered with specific time windows and may only be available on those less busy flying days. Even more extreme, but increasingly common, are super-low fares emailed out to entice frequent flyers to take last-minute trips.

"It's all about supply and demand," Arguelles says. "If they all of a sudden see that they have enough passengers that they can't cancel, but not make money, then they may look at doing a sale on that flight."

For some people, the pursuit of the best fares remains frustrating. Carolyn Stys, 47, of Great Falls, Virginia, has been trying to win the battle of getting the best fare to get her son to and from school at St. Andrews in Scotland for more than two years. It has been one failure after another trying to get the lowest advertised fare. "I've never been able to get lucky," she says. "We (she and her husband) have two laptops with two different sites on each one. Finally, my husband said 'I give up - just book the ticket.'"

(The author is a Reuters contributor. For more from Mitch Lipka see blogs.reuters.com/mitch-lipka/)

(Follow us @ReutersMoney or here. Editing by Beth Pinsker Gladstone and Andrew Hay)

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