What everyone gets wrong about buying cheap airfare, according to the 'Man Who Flies For Free'

10 to 20 years ago, it was common practice to leap on last-minute or standby fares that were dirt cheap.

The airlines were trying to get rid of unused inventory, and people were able to fly for very affordable prices.

But according to Scott Keyes, a reporter for Think Progress and author of "How To Fly For Free" and "How To Find Cheap Flights," this is no longer the case.

"That used to be true back when airlines would want to get rid of their last-minute inventory and just kind of take whatever they could get," Keyes told Business Insider. "But the airlines wised up recently."

What they realized, according to Keyes, was that the majority of travelers buying those last minute seats were businesspeople who needed to book flights to major meetings or conferences - events they'd be willing to shell out a lot of money not to miss.

And because their companies were usually paying for the trip anyway, they were less sensitive to prices and a lot more likely to book pricey last-minute flights.

"These men and women are the ones buying in the last couple days before the flight so the prices tend to go up actually a lot higher in the last few days before a flight rather than dropping off," Keyes said. "So waiting until the last minute for a lower price is a strategy I would vehemently recommend against."

If you're one of those people that keeps hoping to find an affordable standby ticket to a destination, don't hold your breath.

Instead, Keyes recommends buying tickets about three months in advance.

"Wait until between three months to a month out because that's when you tend to get the best prices for domestic flights," Keyes told us.

That's for off-peak flights. If you're hoping to fly to Europe, book a trip home during the holidays, or fly during a time period where a lot of other people will be booking (like for spring break), Keyes suggests looking at flights about six months out.

For more of Scot Keyes's travel tips and advice, check out his ebooks e-books "How To Fly For Free" and "How To Find Cheap Flights."