How do airlines decide who gets those coveted upgrades?

Each Friday on msnbc.com’s Overhead Bin blog I have the pleasure of tracking down an answer to a reader’s questions. This week the topic was: how do airlines decide who gets those much-coveted seat upgrades on the airplanes?

With full airplanes these days, many frequent fliers seeking upgrades find themselves unable to get a spot in the coveted first or business-class cabin.

“I know status plays a part,” noted one Overhead Bin reader. “But many times my husband and I — both Diamond flyers with Delta — have not been upgraded when we know there are seats available.”

This same reader wondered: “Who makes the decision? We have never been able to talk to anyone at the airline that knows — or is willing to tell us — why or how.”

For an answer we turned to Delta spokesperson Paul Skrbec, who told us that a lot of the seemingly mysterious details about how the process works are detailed on the airline’s website. “The IT system uses our published criteria to ensure that the best customers get the best seats.”

After that, and if there are still seats in the front of the airplane, “our agents have the ability to upgrade as well. In these cases, it’s not unheard of for members of the active military to get a better seat if they can be identified in the gate area,” said Skrbec. Other situations are dealt with on a case-by-case basis. “But generally speaking, first-class seats go to Medallion customers and those who paid for first class.”

The criteria are similar at other airlines.

“Procedures are automated to ensure customers have a consistent, fair upgrade experience,” said Rahsaan Johnson, spokesperson for United and Continental airlines. “When space is available, we offer upgrades first to customers in the highest premier levels, taking into account the fares they paid.”

For those without top-shelf, frequent-flier status, though, there may be some secrets — or at least a few strategies to try.

Some travelers believe gate agents often upgrade travelers who ask politely and/or are dressed to the nines. Others say volunteering to be bumped on an overbooked flight, offering to change seats so that a family can be seated together or flat out flirting is the way to win the upgrade game.

And then there’s John DiScala’s upgrade “secret.” The founder of the JohnnyJet.com travel website says the best way to get upgraded is “be genuinely nice and bring a box of chocolates to the gate agents and flight attendants.”