Suzanne Asbury-Oliver, 53, still gets wistful when she talks about her former employer, PepsiCo. “It was a whole different era when we worked for them,” she says. “It was easy, or much easier, to stay busy. We toured for 33 weeks in a row. We had nine months solid on the road.” But ever since Pepsi retired their skywriting program in 2000, after almost seven decades, Asbury-Oliver and her husband and co-pilot, Steve Oliver, 66, have had to work a little harder to pay their bills.

Today, she says, they do maybe 15 corporate events a year, as well as hundreds of freelance jobs. “We do everything from marriage proposals to commercial stuff,” she says. They’ve announced a tour for Lady Gaga in the skies of Miami and given a congratulatory message to a BMX race champion in the horizon of San Bernardino, Calif. Asbury-Oliver doesn’t have exact numbers, but she guesses that in a typical year they write 500 sky messages in more than 150 locations nationwide.

It helps that they have little or no competition. Asbury-Oliver is often touted as the nation’s only active female professional skywriter, but she and Oliver aren’t unique merely because of her gender. “If you Google skywriting,” says Oliver, “there are about a half-dozen places. But basically they’re all brokers and when they get a contract, they call us and we do the job.” There are, Oliver and Asbury-Oliver admit, several professional skywriters out there, but few work with any consistency. “They do this on the side,” Asbury-Oliver says. “Because you can’t make a living at it. We’re the only people who’ve made an actual career out of skywriting, at least these days.”

It helps that the couple have a unique plane, a 1956 de Havilland Chipmunk that they modified with a 27-gallon smoke oil system, which allows them to write about 25 letters in a flight. “Most people,” Asbury-Oliver says, “have a 10-letter limit at most.” Her husband says skywriting isn’t just about having the right equipment. “It requires an incredible level of precision flying,” Oliver says. “There are so many factors. Where’s the sun, which way is the wind blowing, where’s your crowd? You only learn that over time, after doing it enough. And you can’t go practice when skywriting fluid is $10 a gallon and gasoline is $7 a gallon.”

It’s also difficult to learn the craft of skywriting when nobody is willing to teach it. Oliver and Asbury-Oliver won’t share any of their secrets. In fact, while Asbury-Oliver was employed by Pepsi, she was contractually forbidden from discussing her skywriting techniques with anybody, including friends, colleagues, and journalists. “In the ’30s and ’40s, there were a lot of skywriters trying to make a living,” she says. “So they shared nothing. They wouldn’t tell each other what kind of fluid they used to make the letters, what kind of exhaust pump they used. Everything was top secret.” While an air of secrecy might’ve made sense a century ago, why do they continue to be clandestine, despite their competition being essentially nonexistent? “We’re old enough and we’ve been around long enough that we’re both traditionalists,” says Oliver.

Asbury-Oliver and Oliver met at the Kentucky Derby in 1981, where she was skywriting for Pepsi and he, in a lucky coincidence, was towing banners for the same company. It was her first gig for Pepsi, having applied for the skywriting job for lack of any other aviation career prospects. The daughter of a friend, Oliver says, told him: “‘You’ve got to come meet this girl. She’s young and cute and her airplane is nicer than yours.’” He asked to see the plane, and 10 minutes later he introduced himself to the pilot. They hit it off immediately—she’d been flying since 14 in Oregon, and he was, in his words, “an old-time country boy crop-duster pilot” from Missouri. They got married the following year. Asbury-Oliver eventually taught her husband to skywrite, despite her code of secrecy. “The first skywriting I ever did was at the Daytona 500,” he says. “With a half-million people looking at me.”

In their years of skywriting, they’ve done pretty much everything, from beer mug shapes for Oktoberfest parties to “cartoon clouds” at artist gallery openings. “At one point we had a contract with a Japanese or Chinese company—I forget which,” says Oliver. “We had to figure out how to spell whatever this Japanese word was, with the little lines and angles going every which direction. We did it perfectly.” But their expertise doesn’t come cheap. The actual skywriting part, Asbury-Oliver says, isn’t the most expensive part of their service. “It’s getting the airplane to the job,” she says. “It costs about $2 a mile to move the airplane. If we’re in Tucson, Ariz., and they want something in San Francisco, we have to charge them for mileage. And then the skywriting fluid itself is very expensive. It’s like $10 a gallon.”

Every job is different, she says, and almost always more expensive than potential customers think. “People call us because they want to invite their girlfriend to the prom or whatever,” Asbury-Oliver says. “They think it’s going to be a couple hundred bucks. But then I lay out the expenses and explain to them that they’re looking at multiple thousand dollars. That usually scares everyone away.” If that’s not enough to do it, there’s the discouraging prospect that their message might not be seen at all. “We need the right weather conditions,” Oliver says. “We can’t guarantee that it’ll be visible at 2 o’clock on Friday. We need at least a three-day window to get their message up there.”

That’s the bad news. The good news is that customers get a lot of bang for their advertising buck. Oliver and Asbury-Oliver have many stories about skywriting messages that have remained intact for entire afternoons, shockingly visible over multiple city skylines, and drifted hundreds of miles between multiple cities. “It stops traffic,” Asbury-Oliver says of her skywriting creations. “When people see it, they literally slam on their brakes in green lights and stick their heads out the window.”

Oliver and Asbury-Oliver have even occasionally put their own lives in jeopardy. Oliver recalls doing a job for Cindy Sheehan, an anti-war activist whose son was killed in Iraq. She hired the couple to write the words “PEACE NOW” over the 2008 Democratic National Convention in Denver, Colo. “They had a very strict TFR (temporary flight restriction) for the airspace around the convention,” Oliver says. “You could not enter that airspace under any circumstances. You would immediately be met with F-16s.” While he and Asbury-Oliver were doing the sky-written PEACE message, he says, he was radioing the security team of presidential candidate Barack Obama, just to make sure they weren’t in any danger. “During our first pass, we were about a third of a mile away from the restricted area,” Oliver remembers. “And the secret service guy I was talking to was like: ‘Watch yourself. You’re reeeeeeeal close.’ I was like, ‘I know, I know, the turn’s coming up real soon. Just trust us.’”

The couple weren’t shot down by F-16s. Which, in their line of work, counts as a good day at the office.

(This story originally appeared, in a slightly different form, on the Bloomberg Businessweek website.)