(Money Magazine) -- When you hear "JetBlue," "AirTran" or "Southwest," you think "cheap fares." These and other low-cost airlines have rapidly expanded in the past six years, bringing bargains to more markets and burnishing their reputations as the value-minded traveler's best friend.

Lately, though, the low-fare club has been growing and admitting some unexpected members.

Traditional airlines like American, Continental and United are selling more tickets at lower fares, aggressively matching or even undercutting prices on routes where discounters fly.

The result: While your first step when planning a trip used to be finding out whether a discounter served the route you wanted to fly (and then booking without a second thought), your best move today is to search the big boys right along with the discounters.

One reason for the change is that traditional airlines have been slashing expenses (many in bankruptcy court) and are returning to profitability, enabling them to better compete with their low-cost rivals.

As for the discounters, well, their prices can go only so low. If you fly between New York and Los Angeles in early November, for example, you'll pay $337 on JetBlue but just $284 on American, Delta or United. A round-trip Hartford-to-Fort Lauderdale ticket bought in September for November travel cost $314 on Southwest and $293 on Delta.

Southwest and other low-cost airlines still offer more seats at lower prices than traditional carriers do. "The problem is, those seats sell out the quickest," says Southwest marketing vice president Kevin Krone. Therefore, which airline has the lowest fares at any time is partly a matter of timing. And no one disputes the dramatic downward effect that low-cost airlines have had on fares.

But Steven Morrison of the Journal of Transport Economics and Policy says the difference between traditional and low-cost airlines has eroded. "The effect of Southwest on fares today is less than it used to be," he says.

The discounters still have some of their charm: Most cap their highest fares, meaning you'll never be fleeced for $1,200 even if you book a ticket at the last minute, and they have newer planes and fewer restrictions.

So if you want to change your flight, you'll pay nothing on Southwest and $25 on JetBlue. Most major carriers charge $50 to $100. Still, now that finding a cheap fare requires more than just a quick check of one or two discount airlines' Web sites, how do you find the best rate?

Go to the source Airlines save the best deals for people who go directly to their sites. Southwest, for example, reserves some of its lowest fares for its Ding program, which alerts you to a discount with a noise on your computer that sounds like the signal heard when the captain turns off the seat-belt sign.

And all the major airlines offer weekly Web specials. If you buy directly, they don't charge a booking fee or prevent you from earning frequent-flier miles, as some third-party sites do.

Time it right Fares are loaded into reservation systems three times daily but only once a day on weekends. Many airlines sneak in sales on Friday nights, so Saturday is a good time to shop, says George Hobica, who runs AirfareWatchdog.com, which scours airline sites for hidden deals.

Plan ahead Though many airlines offer low fares for last-minute trips, they are often on unpopular routes and at inconvenient times. The best way to save is to buy ahead - remember that most seats go on sale nearly a year in advance. How does Florida next November sound? Book now.

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