Ten years ago, Welch was gearing up to speak at a leadership conference in Toronto, Canada, as part of an international book tour. There were 5,000 people in the audience.

But bestselling management author and CNBC contributor Suzy Welch says that no matter the nature of your message or the size of the crowd, there's one fool-proof tip that can help anyone deliver a great speech: Speak from the heart.

Should you rehearse a ton, and risk sounding scripted? Should you speak off the cuff, and risk getting tongue-tied? There are tons of factors to consider — including the size and temperament of your intended audience — and it can be tough to know what tone to strike, especially in a situation where you suspect you'll be nervous.

Public speaking can be frustrating, anxiety-producing, or, for some, downright terrifying.

"I'd been on the road for eight weeks," Welch tells CNBC Make It. "I missed my husband and my kids. I was exhausted. I should have gone home." The book she was promoting was all about creating a life with balance, but admittedly, Welch says, "my own life had gotten away from me."

"As I walked out on the stage, I looked at the audience and thought, 'I just can't do this. I've got nothing left.' I heard myself asking for a chair, and when it arrived, I sat down in the middle of the stage."

Rather than delivering her carefully-crafted speech, Welch says she "just started to speak from my heart." She told the crowd, "I was supposed to talk to you today about work-life balance, but here's the truth: I've kind of lost mine."

Welch delivered the key parts of her prepared speech, but says her words were "fortified with more authenticity, pain and humor than usual."

To her surprise, she says, it was her most successful speech ever.

"I learned a valuable lesson that day," she says. "A speaking persona, no matter how impressive, builds a wall between you and your audience. My advice: If you have one, kiss it goodbye."

Instead, Welch says, you should "compose every speech as if you are presenting your ideas to friends."

"Assume intimacy," she emphasizes. "Assume trust. And then talk that way."

This advice is not, Welch warns, license to "wing your next public speaking engagement." Your presentation "still needs great content, data and stories." But keep in mind that "people don't respond to slides or concepts or polished delivery — they respond to people. They respond to real people."

"Whether you're speaking to an audience of 5,000 or five, take down the wall and open a door," Welch says. "The audience will walk right in."

Suzy Welch is the co-founder of the Jack Welch Management Institute and a noted business journalist, TV commentator and public speaker. Think you need Suzy to fix your career? Email her at gettowork@cnbc.com.

Video by Beatriz Bajuelos Castillo

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