All you have to do is buy a large bag of M&Ms, or a similar bag of giant, shareable sweets.
"I take somewhere between 200 and 250 flights per year," Mr Shankman writes in his blog.
"And no matter what airport I'm in, no matter whether I'm flying 200 miles to Boston or 12,000 miles to Singapore, I won't get on the plane without stopping at the convenience store in the airport and buying a big bag of M&Ms.
"I walk onto the plane and find the lead flight attendant. I hand him or her the bag, and simply say 'These are for you, but you have to share with your co-workers, OK?'
"Then I smile at the lead flight attendant and walk to my seat. That's it."
It doesn't matter if you're in first class, business or the worst seat in economy. Spend some loose change on a big bag of chocolates, and the results will be instantly uplifting.
Mr Shankman emphasises that this isn't about sucking up to get a better seat or free drinks. Rather, he says, it's a brief moment of joy that inevitably filters down from the pilot to every last passenger.
"When you walk on, no matter how much the flight attendants smile, there's a subconscious wall of protection that goes up, and it's subconsciously noticeable by everyone else. It's not intentional, it's just what it is, from years of FAs having to put up with abuse, and years of passengers feeling abused. It's a vicious cycle, that won't ever stop."
He says that, when he gives them the M&Ms and tells them to share it with the crew, their reaction always goes the same way: confusion and disbelief, followed by a massive smile.
"The payoff is huge," he writes. "The flight attendants are happily munching on candy for the entire flight, they're smiling, and their good mood is infectious! They're joking with the passengers, they're cheerful, for possibly the first time in a while, they don't feel like every passenger is out to get them!"
Mr Shankman's post has been flooded with positive comments - some from passengers who tried out his tip and succeeded, and others from flight attendants who vouch for it.
"On behalf of flight attendants everywhere, thank you!!!" wrote one user. "I can attest to everything this post says is true. It's especially true for short haul domestic flights. Many times we work three or four flights a day with very little time in between. An extra treat that shows you care about can make our day just as the guy who comes on and complains surely puts a damper on it."
Another user spoke of how a similar tactic - buying the staff at a maternity ward pizza - had the same effect.
A little kindness goes a long way!
