NEW YORK (Fortune) -- No question about it: Plenty of managers in their 30s and early 40s - that restless group known as Generation X - are frustrated. Boomers are taking up all the plum senior jobs and, in many big companies, seem in no hurry to retire.

The era of tedious dues-paying to earn career advancement is long gone and, for the cohort who grew up during the have-it-all Eighties and joined the workforce in the go-go late '90s, waiting around for a chance to move up is just maddening.

If that describes you, two words: Don't wait.

"If you're in middle management at any age, but especially in your 30s, you have a big decision to make. Do you really want to break into the senior echelons?," asks Phylis Esposito. "If so, you have to find a way to stand out. You have to get more visible, which usually means taking a risk - sometimes a big risk. It's tough, because you may even have to put everything you've already earned on the line in order to do it."

Esposito knows whereof she speaks. Now executive vice president and chief strategy officer (one rung below CEO) at Omaha-based TD Ameritrade, Esposito catapulted into top management about 15 years ago. While still in her early 30s, she left Goldman Sachs, where she had been the first woman to run a trading desk, and co-founded an investment firm called Artemis Capital.

"I risked everything I had gained up to that point to do it, including a secure, high-paying job with lots of prestige. It was a little scary," she recalls. "But it made me immediately visible. Starting your own company gives you immediate access to other senior people. I could pick up the phone and call any other CEO on Wall Street."

Artemis was profitable in its first year and, once she and her partners had sold it, the investment-banking experience Esposito had gained there vaulted her into a managing director job at Bear Stearns.

Of course, not everyone is cut out to start an enterprise - and that's okay. Finding ways to expand your skills and get broader experience in the company where you already work can make you more visible and lead to a bigger job, too. Just ask Steve Lewis, 37, who has held 10 different jobs in 16 years. Among many other projects, he's helped start an insurance company from scratch, worked on several mergers, headed up the 24-month turnaround of a troubled subsidiary, and had a hand in developing Asia strategy.

Most of his moves have been lateral ones, all within Zurich Financial. "To move up, move left, then right, then left again," Lewis says. He recently joined the senior management team as chief financial officer for European Global Corporate operations.

"Most companies have loads of opportunities if you really look for them and you're prepared to be flexible," Lewis says. "The thing is, people often box themselves in to the point where they can't even consider making a change. I know people who won't even move 50 kilometers down the road, let alone overseas." Clearly not an issue for him: Lewis has moved eight times, from the U.K. to Hong Kong, back to the U.K., then to Sydney, and now lives in Zurich.

But it isn't the number of stamps on his passport that has gotten Lewis into the C-suite - it's his willingness to take each narrowly defined job he's been given and make it...bigger. "Call me nosy, but I like to get involved in areas adjacent to mine," he says. "You start by doing the basics really well, then you identify where the gaps are, and deliver more than what's expected."

Isn't that a bit risky politically in most companies, where managers tend to guard their turf? Yes, but charm helps. Notes Lewis, an affable Brit, "Obviously you need diplomacy and tact, so you don't rub people up the wrong way."

You need a strong network of friends and allies, too. We've all heard and read plenty by now about the importance of networking, although it is one of those notions that often seems honored in the breach. Consider a recent survey of 1,805 managers by ExecuNet, an online career-services firm: 84 percent of those polled said they believe a broad network of personal and professional contacts is essential for success in business. Yet only 19 percent said their own networks are in "good" or "excellent" shape.

Phylis Esposito counts herself among the many senior honchos who owe their current lofty status to a well-timed word of advice from a peer or a pal. "You must take risks to get ahead, but some risks are just stupid," she notes. "A good network can really help."

Esposito belongs to a women's networking group called 85 Broads - the name is a play on Goldman Sachs' Manhattan street address - that was started in 1999 by fellow Goldman alumna Janet Hanson and has since grown to more than 10,000 members worldwide.

"At every step of my career, I got solid advice from Janet and other women who knew Wall Street," she says. "You need to be able to bounce ideas off someone you can trust. Sometimes the best advice you get from a mentor or a peer is not about making the right move so much as it is about steering clear of the wrong one."

The worst move you can make, however, is none at all. "Doing nothing and just hoping your next promotion will somehow take care of itself is really the biggest risk," says Esposito. "Don't forget that there are people coming up behind you who want your job."

Indeed, if you're a Gen Xer who feels outnumbered and outgunned by Boomers, just wait 'til you meet Generation Y: Almost as numerous as their Boomer parents, loaded with energy, tech savvy, and ambition, and poised to blow your doors off unless you hustle to stay one step ahead.

As Will Rogers once said, "Even if you're on the right track, you'll get run over if you just sit there." Too true

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