Getting a free upgrade on an airline is getting more difficult, but it’s still possible if you know what to do. 

Last year more than 742 million passengers flew on domestic air carriers, breaking the record set the prior year by more than 20 million flyers, according to the Department of Transportation. On average, airplanes flown by domestic carriers in 2017 took off 85 percent full, up from 80 percent a decade ago.

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According to a recent airline satisfaction survey of more than 55,000 Consumer Reports members, two major complaints for economy-class fliers were that seats were uncomfortable and legroom was insufficient. 

Gary Leff, who runs the website A View From The Wing, says that asking for an upgrade politely and wearing business attire used to be common advice for success when there were more seats available. But these days, you may need to do more than that if you want to fly more comfortably.

To increase your odds of getting a better seat—without paying more—try following these tips:

Check With the Gate Agent

Gate agents may be able to grant a flight upgrade if the coach section is overbooked. Leff says that their most important task is to get the plane off on time, so if there are seats available in the front sections of the plane, they may upgrade people during boarding.

To get one of those seats, arrive a bit early at the gate, when the agents are a little less frazzled, and tell them politely that if they are upgrading passengers you’d like to be considered. Then wait at the gate even after your row is called so that you are one of the last to board. “They won’t want to take the time to get someone off the plane to upgrade them,” Leff says.

The trade-off, Leff notes, is that if you don’t get upgraded, the space in the overhead compartments near your seat may be full by the time you board and you’ll have to check your bag.

Avoid Competing With Business Travelers

To improve your chances of getting an upgrade, fly when business travelers are less likely to be in the sky. For example, avoid early Monday morning flights, when most business travelers leave on trips, as well as Thursday and Friday evening flights, when they tend to be returning home.

Your chances of getting an upgrade are best on Tuesdays, Wednesdays, and Saturdays, when planes are emptier, Leff says.

Fly a Foreign Airline

If you are flying internationally, there are likely to be a lot more available seats onboard a carrier from another country. Foreign carriers’ flights were only 62 percent full last year, on average.

If you’re in a frequent flyer program with a U.S. airline, see whether it belongs to an alliance that includes foreign carriers. For example, American Airlines belongs to the One World alliance, so if you have frequent flyer points on American, you could use them to book a flight on another airline that is part of the alliance.

For One World, that would include Air Berlin, British Airways, Cathay Pacific, Iberia, and Quantas, among others. United Airlines is part of the Star Alliance, which includes Air Canada, Air New Zealand, Lufthansa, Scandinavian Airlines (SAS), Singapore Airlines, and Swissair.

—Amanda Walker contributed to this article.