People who frequently travel by air have a number of ways to save money when booking flights.

But The Points Guy CEO Brian Kelly says there is one mistake many passengers make when flying with Delta Air Lines  (DAL) , United Airlines  (UAL) , and American Airlines  (AAL) .

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One method travelers use to save money involves a willingness to be flexible on the specific days of travel. Changing departure and return dates by just a few days on either end can significantly reduce costs.

Many people also save money by exploring discount airfare websites and by checking with airlines directly.

Another way to find less expensive fares is to consider longer flights and layovers. Travelers also can investigate whether booking one leg at a time results in spending less than buying a round trip ticket.

And a large number of travelers leverage frequent flyer airline miles and credit card points for discounted airfare.

Now, in an exclusive interview with TheStreet from the floor of the New York Stock Exchange, The Points Guy founder and CEO Brian Kelly explained a mistake these travelers often make.

The trick to avoid making one air travel mistake

“What are some of the most common mistakes that you see people make?” asked host J.D. Durkin.

“The mistakes I see people make are not accruing miles,” Kelly said. “Or, a lot of people will say, ‘Oh, the frequent flier mile game, it’s so expensive these days.’ And that is true to an extent.”

Kelly shed some light on a strategy people can use regarding booking flights on major U.S. airlines.

“So the big three airlines — Delta, United and American — they have increased the amount of miles you need to go, sometimes eye-poppingly,” he said. “So Delta is known for having 300,000 miles-plus one way to Europe.”

“But here’s the trick, people,” Kelly continued. “Don’t put all your miles and points in those airlines. Real people in the know use credit cards. Be loyal to your credit card that allows you to transfer to a lot of different partners.”

Kelly explained this further using an example involving Delta and Virgin Atlantic.

“For example, if you want to fly Delta — you can fly Delta business class — that same 300,000 mile ticket, you can get as low as 50,000 Virgin Atlantic miles.”

One Google search can save travelers money

Kelly discussed what he called a “hack” people can use when looking online for airline tickets.

“So there’s a hack to using Virgin Atlantic,” Kelly said. “You can, if you just Google ‘Virgin Atlantic award calendar trick,’ you can snatch Delta flights super, super cheap.”

“So it’s earn points in the U.S. where it’s easy to earn points,” he added. “And then transfer them to foreign frequent flyer programs: Air Canada Aeroplan, Air France Flying Blue, Virgin Atlantic. And snag deals with those programs because they have lower thresholds for the amount of miles needed.”

The Points Guy is a travel website that was founded by Kelly in 2010. Initially, it was a blog that was written by Kelly. Now, it employs a full-time staff and also publishes stories written by freelance travel reporters.

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