First, it was getting tickets before your flight was two weeks away. Then, it was avoiding a Saturday overnight in your travels. Many rules that helped savvy travelers keep their costs low have gone away. The latest casualty, though, is a doozy: It's now cheaper to buy a bunch of one-ways, called segments, rather than a multi destination round trip. Huh?

According to the Wall Street Journal, American, Delta and United now don't include their cheapest prices in multi city trips. If you leave London to have a meeting in Brussels and then head to New York, then you're now better off buying each individual leg rather than doing it as one trip. The difference could mean hundreds, if not thousands of dollars.

WSJ's Scott McCartney puts it best: "Think of it as making a six-pack of soda twice as expensive as buying six cans individually."

There are a few reasons why the biggest airlines would take this counterintuitive step: They've made flights too cheap competing with Spirit, Frontier and other carriers; They can actually make less money on single-flight itineraries; and, paraphrasing a statement from American, to give local passengers a better chance of getting cheaper fares usually absorbed in multi-city itineraries. 

Here's what you need to do now:

Tell your travel agent: If you have one, make sure she knows the update and checks for your fares as individual flights rather than whole. This step is even more crucial if you use multipurpose agencies like AAA that don't exclusively do airfare - they might not have heard about the change.

Buffer in more time for booking: A multi-city itinerary can be complicated and time consuming, even when you are doing it as a single purchase. Now, to maximize your value, you need to view it as booking each city stop separately, so a three-city trip will take as much time as organizing four individual flights (the three arrival cities and the return home).

Space out your layovers: In a traditional multi-city itinerary, your next flight will be given a heads-up if you encounter any flight changes or delays. That's not the case with a bunch of one-way flights - you're on your own! Keep the space between landing and leaving as wide as possible to allow room for unexpected challenges.

Consider smaller airlines: Put Southwest, Virgin/Alaska, and other reputable carriers on speed dial, as the change isn't a universal law - it is just the decision of the three biggest airlines. The little guys have their own benefits, too, like Southwest's extremely flexible flight changes and Virgin's high-end coach cabin experience.