FORTUNE — Dear Annie: I work on a team with a bright, talented young woman who has a lot of potential. The problem is that our director and other colleagues are frustrated with her communication style, which is what you might call “Valley Girl.” We really want her to do well and get ahead, and we believe the way she speaks is holding her back. Can you offer any tips on how to overcome this? She is generally open to constructive suggestions and I think she would follow your advice. — Trying to Help

Dear TH: “’Valleyspeak’ is the verbal equivalent of coming to work looking like you just rolled out of bed,” says executive speech coach Christine K. Jahnke. “It’s sloppy and, worse, it distracts people’s attention from your ideas and your performance. It can also wreck your chances of ever being selected for a job where you would be ‘out front’ dealing with clients.”

Jahnke is president of Washington, D.C.-based coaching firm Positive Communications and has advised Michelle Obama and six state governors as well as executives at companies as diverse as the National Geographic Channel and NASCAR. She also wrote a new book, The Well-Spoken Woman: Your Guide to Looking and Sounding Your Best.

One drawback to talking like a Valley Girl is that it often entails ending sentences with an upward inflection, as if they were questions, which “sounds as if you’re seeking approval rather than making a statement,” Jahnke says. “It makes you seem to lack confidence in what you’re saying.”

Another unfortunate verbal habit: Peppering one’s speech with “like” and “you know.” Jahnke believes that Carolyn Kennedy’s 2008 Senate bid failed in part because of public appearances — including a New York Times interview that quickly went viral — where, Jahnke recalls, Kennedy “seemed unable to articulate a complete thought without saying ‘you know.’” Pundits also picked on Kennedy’s “baby doll voice” and “tendency to ramble,” Jahnke says.

Of course, most of us (luckily) never have to stand in the intense public spotlight that candidates for public office face, but a less-than-polished speaking style can wreck anyone’s career prospects, says Jahnke, and it becomes more of a sticking point the higher you go: “As you rise up the ladder, expect that every aspect of your speaking persona will face more intense scrutiny.” Gulp.

So what can your colleague — or anyone else — do to change speech patterns that may have become ingrained over many years? “Most people really don’t need a total overhaul,” Jahnke says. “They just need to correct one or two things.”

She suggests the following four steps toward a more professional speaking style:

1. Seek out honest feedback. As with trying to change any behavior, the first step is to become aware of it. In her coaching sessions, Jahnke usually starts by videotaping a client talking and reviewing it with the client. “People are usually surprised when they watch and hear themselves,” she says. “Most of us don’t really know how we’re coming across.”

A trusted friend or coworker may be able to offer suggestions, or your company’s human resources department may even provide a few professional coaching sessions — especially for high-potential types like your teammate.

2. Join Toastmasters International. With 13,000 chapters in 16 countries, Toastmasters probably has a club near you. “It is a great organization, full of people who are seriously trying to improve their speaking skills in a friendly, collegial atmosphere,” Jahnke says. “And it’s free.”

3. Study the speaking styles of successful people. “Women have so many more role models now than ever before, so it’s easy to find executives whose speaking styles have helped to get them where they are today,” notes Jahnke. She recommends checking out TED.com, which offers thousands of 20-minute talks by interesting people.

“Look up Sheryl Sandberg at Facebook, or Melinda Gates,” Jahnke suggests. “Or find YouTube videos of [Pepsico CEO] Indra Nooyi. Notice how they pace their speech, and how they use pauses. With some effort and practice, these are things anyone can learn.”

4. Take note of how higher-ups at your company express their ideas. Naturally, communication styles vary somewhat from one corporate culture to another, so it makes sense to pay attention to how people above you talk. “If there is someone who is particularly effective at getting his or her ideas across, you might emulate the way they do it,” says Jahnke.

“The idea is not to parrot someone else’s speech patterns, but to adopt the elements of their style that you can comfortably learn to use,” she adds. “In the end, it’s still about expressing yourself and your unique ideas — but your best self, presented in a way that will make others listen.”

Talkback: Do you agree that the way someone speaks can make or break a career? What habits of speech do you find distracting or unprofessional? Leave a comment below.