If you're planning a summer flight and have your sights set on a window seat or aisle seat, you should prepare to cough up some extra dough.

More and more of the major airlines are now considering those economy seats a luxury and are looking at new ways to charge passengers for them.

Scott Mayerowitz, an airline expert with the Associated Press, told ABC News today that passengers on low-cost carriers like Spirit Airlines had to pay for seat assignments in advance or risk having the airline put them in whatever seat is available.

Other airlines have followed suit, charging fliers in some way for a good seat. Southwest, which doesn't assign seats, recently increased the price of its early boarding by 20 percent. It now charges passengers US$15 each way to board the plane first to improve their chances of getting a good seat.

With Delta's least expensive option, basic economy, fliers cannot select their seats until 24 hours before they travel - just around the time when the most coveted aisle and window seats are inevitably taken by people who paid a higher fare.

"American Airlines and United are following Delta's lead on this and will be launching their own basic economy fares in coming months although they haven't yet released the details of those plans," Mayerowitz said.

He said travelers using the big airlines like American, Delta or United would likely have a harder time finding seats because airlines were reserving more seats in the front of the plane for elite members or those willing to pay extra.

"Regardless of what fare you book," Mayerowitz said, "if you book only a few weeks before your trip, the only seats left are those middle seats or the ones next to the bathroom. ... Fliers should just keep checking and checking airlines websites. Especially at that 24-hour-before-departure time, make sure you check in and look at the seat map. That's when a lot of elite fliers get upgraded and their seats open up and you can jump on in and snag those."

Travel experts said that there was one way to improve your chances at a better seat, though - and without having to pay up.

When consumers book their flights, experts suggest entering the information into ExpertFlyer.com, which automatically emails the traveler when an aisle or window seat opens up.