By Jennifer Reingold
June 12, 2013

FORTUNE–The oil titan — and first U.S. billionaire — John D. Rockefeller was once reportedly asked how much money would be enough. “Just a little bit more,” are the words famously attributed to him. Though it’s nearly impossible to ascertain whether Rockefeller actually spoke those words, it captures the truth for many Americans. The same principle has always held when it comes to professional success. You strive to get there, then you do what it takes to stay there. You simply cannot be too successful, nor can you be successful for too long.

That’s a pretty good summation of how George Stalk once viewed his career. Stalk was an indefatigable, world-renowned strategy consultant and partner at Boston Consulting Group who wrote books like Hardball: Are You Playing to Play or Playing to Win?” and who carried briefcases overflowing with cans of Diet Coke from one client crisis to another.

He was at the apex of his career in 2004, at age 53. Then it all changed. After months of ignoring signs that something was very wrong with him, Stalk was in Boston when a blood vessel ruptured in his stomach. He made it back to his home in Toronto, where his overtaxed body surrendered. He fell into a coma for three months and was declared dead three separate times. George Stalk had succeeded — practically to his grave.

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Improbably, Stalk survived. After months of rehab, he returned to BCG, where he is now a senior adviser, and also works as a senior partner of Banyan Family Advisors, a firm that works with family businesses. He is, once again, satisfied and successful (and healthier, according to his doctor). Yet his definition of success today is far from what it meant a decade ago, when he prided himself on being “anywhere in the world at any time when needed.” Stalk has had plenty of time — much of it in a hospital bed — to ponder the common themes in a successful career, and he knows from experience that no career follows a straight line.

For Stalk, success is still about ideas. But now he paces himself and works only on big subjects, such as supply-chain gridlock and managing in periods of high uncertainty, for people and clients that he likes. He travels far less frequently and has for the first time in his life discovered hobbies — Stalk likes to fly radio controlled model jets — which, he says, have improved his creativity and idea generation. He now counsels his clients that “you need to spend more time out of the office, the higher you get.”

Stalk’s experience is extreme. Yet it is a good analogy for the types of professional shocks we all suffer regularly in this volatile economic environment — and a reminder that success must be defined differently at different stages of a career. James Citrin, co-chair of the North American Board & CEO Practice at search firm Spencer Stuart, writes that it’s helpful to think of most careers as falling into three distinct segments; the “Promise Phase,” the “Momentum Phase,” and the “Harvest Phase.”

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Traditionally, the Promise Phase takes place at the beginning of your career — Citrin says the first five to seven years. During that time, your success is judged on the potential you offer more than the actual value you provide on the job. That changes in the Momentum phase, where your worth is measured by both your expertise and your track record. In the final, Harvest Phase, which Spencer Stuart says typically occurs between ages 45 and 55, there are two groups: those who begin to run the clock out on their careers and those who continue to succeed — by finding ways in which to bring that hard-earned expertise into new situations.

All of this made sense as recently as a decade ago, when there was still some sense of job security for those who worked hard and had successes to trumpet. But now, says Allison Hemming, CEO of digital talent agency The Hired Guns, disruptions and volatility in the job market affect even the most successful. The Department of Labor says that baby boomers now hold an average of 11.3 jobs over the course of their careers. That means that we are all likely to experience several cycles of the three phases — and not necessarily in order. To be successful at every stage, Hemming counsels, you must operate as if you’re in the momentum phase at all times — which means never letting skills decay and always looking to add more. “Momentum has to be about both earning and learning,” she says.

Stalk agrees — and has a few suggestions on how to maintain that momentum, based on both his consulting work and his personal journey. One of the most important, he says, is to “keep an open file.” What he means by that is to always have other interests or projects on the back burner. You may not have the time to devote to them, but if you find information that could be helpful, save it and put it aside until that time comes; you’ll never find yourself without a next new project, proposal, or even career.

Another way to stay relevant is to think of your career as a set of skills rather than a specific profession and to network accordingly — not only with your own peers but with younger people as well. This way, if your industry contracts, you can more easily pivot. You have to constantly be vigilant,” says Hemming, and say, ‘what is the job after this job and what is the job after that? If I’m not here, which company am I at?'”

Finally, to be successful at every stage, you must be flexible in your own definition of success. For Stalk, it changed — from being a sort of corporate Superman to merely being able to read the paper, to, today, finally balancing work, health. and family. “Just a little bit more,” for him, has a whole new meaning.

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