Isn't it frustrating when you finally find a great airfare, but by the time you go to buy it, it's gone?

Sometimes, it's just the last seat at that price being snatched up by another savvy consumer. In other cases, that great airfare may have just expired, been withdrawn or increased.

Some say that if you're searching for fares online, the airlines are tracking your entries, and when you show interest by going back to their site, pow! They hit you with a higher rate. That's a myth. The truth is that the airlines are constantly updating (raising and lowering) thousands, sometimes hundreds of thousands, of fares multiple times a day. Knowing that this occurs, and when, can help you track those changes and give you the edge on getting a great fare.

Domestic fares begin updating at 5 and 7:30 a.m. (all times are Hawai'i time), Mondays through Fridays. On Saturdays and Sundays, these fares are updated only at 1 p.m. International fares are updated at 8 p.m., 12:30, and 6 a.m., and 10 a.m. and 3 p.m. Mondays through Fridays; 8:30 p.m. and midnight and noon Saturdays; and noon and 3:30 p.m. Sundays. (Because updating is complex, there may be no best time to check, although it can't hurt to survey prices late at night and first thing in the morning.)

After updating, fares are sent to reservation systems used by most airlines and travel agents. Because the updates take time, some uploads go first to certain systems, such as Sabre versus Worldspan.

Some fares that seem like great bargains actually are mistakes made by an airline. Other times, they might be retaliation fares that an airline posts as a warning to competitors regarding undercutting. These types of fares are almost always gone by the next update. Some low-fare carriers such as Southwest and JetBlue don't list their fares in all the major systems. You have to go directly to their Web sites to get their rates.

It gets even more complicated. Airlines have very sophisticated yield-management systems that can open or close the availability of seats for each fare offered. And those updates can occur at any time.

So what's the message? Keep checking. And when you see a bargain, grab it.

Tom Parsons is publisher of Best fares.com. Reach him at www .bestfares.com/contact.asp.