Maggie DaRonco scrambled to find another internal spot last fall after her employer, a big European bank, decided to eliminate the project manager’s post in Jersey City, N.J.

She pursued three openings, and all three hiring managers told her, “You’re the leading candidate.” But her hunt still failed. “I was really overconfident,” said Ms. DaRonco, now unemployed after a 15-year banking career. “I thought everyone would want me.”

Being an insider doesn’t guarantee you the inside track to an inside job. Assuming colleagues know you’re terrific is among the most common mistakes made by inside applicants. It’s hardly the only one, however.

Such mistakes explain why fewer than a third of the people seeking alternate employment with their current company get hired, said John Sullivan, a management professor at San Francisco State University. Further lowering the odds, he adds, is the fact that “a growing number of firms now give equal or even superior consideration to an external candidate.” And surveys show that big businesses are filling a lot more jobs with outsiders.

The best solution? Spend more time preparing for an in-house move than for an outside vacancy. Internal hiring managers expect you to grasp every nuance of the business because you enjoy tremendous access to co-workers and corporate data.

“You should have the brass polished when you come in,” Annie Stevens, a managing partner at ClearRock, a Boston executive-coaching and outplacement firm, suggested. “A lot of people do just the opposite because they think they’re a shoo-in for the job.”

In crafting your in-house battle plan, you should investigate a hiring supervisor’s style and needs, other key departmental players, the track record of the latest jobholder – plus your chances for success. Arrange informational interviews with lieutenants of the hiring manager. Gather further intelligence by networking extensively with colleagues outside that department. Ms. DaRonco conceded that she didn’t do any homework about her three would-be bosses. “I could have focused on that more,” she said.

Last year, a project manager at a major pharmaceuticals company rejected five of six internal prospects for a laboratory scientist’s job primarily because they had done so little due diligence. “The message these candidates sent to me was, ‘I’m already employed by this company so I don’t have to try as hard to impress you,’ ” he recalled.

As part of your preparation, you should also figure out which associates are vying for the position. Catalog their skills and experience, Mr. Sullivan suggested. “Then, develop specific arguments as to why you are a superior candidate in every key skill, education and experience category.”

Developing those arguments will help you revise your resume, too. Test market the improved version by posting it anonymously on job Web sites and seeing how many employers respond.

You should also alert your current supervisor. Otherwise, he could sabotage the transfer by providing a bad reference or insisting he can’t afford to lose you. Or worse: A Detroit assistant store manager for a home-improvement chain sought a job in the loss-prevention department. He got fired after he skipped work for two days of skills tests without telling store management why. Loss-prevention officials hastily arranged his reinstatement and transfer.

It’s also foolhardy to cite your superior as a reference without making sure your manager will say good things about you, said Kristen Fife, a recruiter at Volt Technical Resources at Redmond, Wash.

She learned that lesson the hard way. A Seattle training company hired Ms. Fife to be a customer-service team leader in fall 2001. Things didn’t work out. A month later, her boss gave her the choice of a demotion or an in-house job hunt. “She led me to believe she was giving me a good reference,” Ms. Fife recalled. Instead, she says, the woman told a hiring manager “that I didn’t know what my skills were.” Ms. Fife recommends that internal applicants request written performance feedback from their supervisor.

Your real opportunity to shine is during the job interview. Because your interviewing skills are probably rusty, videotape practice sessions. And avoid the mistakes many insiders make: Dressing too casually, criticizing their upper-management sponsors, or failing to exude enthusiasm about their qualifications.

Kim Fous, an insurance-company human-resources director until late 2003, often interviewed potential middle managers who hadn’t assembled resumes and supplied applications that were blank except for their names and titles. They expected to be picked because they were “so respected by the management team,” said Ms. Fous, currently an account manager at recruiters Angott Search Group at Rock Hills, Mich.

Even if your interview goes well, don’t boast to co-workers. “It gets back to the hiring manager,” warned Elizabeth Ryan, a former HR executive who’s now CEO of WorldWIT, a global professional women’s network. “I’ve seen internal candidates blow it by telling colleagues, ‘I have a lock on the job.’ “