(FORTUNE Magazine) – Monsanto CEO Bob Shapiro leans across his black marble desk in Creve Coeur, Missouri, and extols that ultimate hierarchy buster, E-mail. "I love it," he says. "I'm directly connected with 25,000 of our 30,000 employees worldwide. I tell them that if you come up with a good entrepreneurial idea and you can't convince anybody in the organization that it's worthwhile, just drop me an E-mail." He vows to pursue all promising ideas--for new products, new processes, new services.

So do the chiefs of the few other famously exceptional companies where determined individuals regularly move their ideas up the hierarchy and into the market. Outfits like Hewlett-Packard, Microsoft, Intel, and Wal-Mart have this rare record because they themselves were founded by iconoclastic entrepreneurs. Others, like General Electric, have charismatic leaders who really swear by in-house venturing. To this day employees at 3M recite founder Bill McKnight's mantras: "Listen to anyone with an original idea, no matter how absurd it might sound at first." "Encourage. Don't nitpick." "Give it a try--and quick!"

If you are fortunate enough to work for such a company, you don't need Santa Claus. Trouble is, most corporations aren't like that. When it comes to entrepreneurship, they talk a great game but do a lousy job. Tirelessly they tell us they want employees to push through ideas, yet their bureaucracy has a marvelous ability to snuff them out. "The problem with hierarchies," says Bell Labs chief scientist Arno Penzias, "is that people at every level have the power to say no. You have to get to the person who can say yes."

So what should you do if you're a born entrepreneur who's doing time in a corporation? How can you break through those walls with your great idea?

Most important, you must find allies--not only inside but also outside the company. If your idea is languishing on your boss's desk, take it to another department or division within the company that might pursue it. Or persuade trusted customers--who really do get listened to--to ask the company why it can't supply just what you are proposing. Include their requests for your product, their research showing there's a market for it, and their testimonials in your written proposal to top management.

"Writing and verbal skills are critical," says CNBC president Roger Ailes, an entrepreneur and former Republican media consultant who created the America's Talking network out of thin air. "You have to be able to crystallize your idea in a memo, spelling out your vision in a dramatic way."

NYU business professor Zenas Block, co-author of Corporate Venturing (McGraw-Hill, $36), suggests that you answer these questions: Why is the idea good? How does it fit in with our corporate strategy? Is it worth the risk and cost? What resources and skills do we need?

But forget about cobbling together extensive business plans that project future losses and profits. Block calls them "exercises in fantasy, quantified by Lotus." (The spreadsheets always show profits in year five.)

In addition, the professor offers some valuable principles for getting a new idea through the corporation:

Never ask for a decision any larger than you need to go on to the next step. The larger the decision you ask for, the more threatening it is to whoever has to make that call. Rather than trying to sell a full-blown but untested idea, sell the simpler notion of getting a small amount of money to check out whether it can fly. Take it step by step.

Undercommit and overperform. If you think you can reach $4 million in sales by the end of year one, but can get approval by promising to reach $2 million, then commit to the two but hit the four.

Avoid premature publicity. Internal publicity leads to jealousy and often to early disappointment. External publicity just advertises your venture to competitors.

Recognize your own weaknesses. If you're good at marketing but terrible at finance, go to a higher-up and get somebody on your development team who's expert at finance.

If all else fails, you may have to find a venture capitalist and quit the corporation. If you then succeed, the odds are strong you'll be able to sell your hot idea right back to your old employer--for a lot more than it would have paid if the company had underwritten you.

In any case, be prepared to be unpopular. When asked how you identify the entrepreneurs in a company, business philosopher Peter Drucker responded, "Look for the troublemakers." Or as George Bernard Shaw put it, "All progress is the result of the efforts of unreasonable men."

Reporter Associate Melanie Warner