Billionaire investor Warren Buffett says the very best investment you can make is one that "you can't beat," can't be taxed and not even inflation can take away from you.
"Ultimately, there's one investment that supersedes all others: Invest in yourself," Buffett says in a recent interview with Forbes. "Nobody can take away what you've got in yourself, and everybody has potential they haven't used yet."
One of Buffett's investments in himself came in early adulthood, when he signed up for a $100 Dale Carnegie public speaking course that he says changed his life.
"I was terrified of public speaking when I was young. I couldn't do it," Buffett says. In fact, he admits he would become physically ill when the time came to take the podium.
The course was taught at Dale Carnegie, the institute named for the influential speaker and author of "How to Win Friends and Influence People."
"You can't believe what I was like if I had to give a talk," Buffett recalls in the biography "The Snowball: Warren Buffett and the Business of Life." "I would throw up."
Even so, the young investor was determined to get over his crippling fear of public speaking and to woo Susan Thompson, who he would later marry.
"I proposed to my wife actually during the middle of the course, I got so confident about my abilities," Buffett tells Forbes. "It also helped me sell stocks in Omaha, despite being 21 and looking even younger."
Buffett tells Forbes that you can exponentially increase your potential "by simply being able to communicate better" and "enhancing your talents."
Your potential is something that will help you have a more interesting life, Buffett adds, but there is no better time to work on that than today.
"Address whatever you feel your weaknesses are, and do it now. Whatever you want to learn more, start doing it today. Don't put if off to your old age," Buffett says.
"You'll have a more rewarding life not only in terms of how much money you make, but how much fun you have out of life; you'll make more friends the more interesting person you are," Buffett says. "So go to it, invest in yourself."
Today, would-be orators can hone their skills in the same class where Buffett developed his.
CNBC sat down with Joe Hart, president and CEO of Dale Carnegie, to discuss some of the most important lessons professionals can use to become better communicators — the same ones that that changed Buffett's life.
"One of the traits that is awesome about [Buffett] is he is such an active learner," Hart tells CNBC. "It's something that's made him very successful."
Because Buffett is always learning, he's continuously building up the cache of topics on which he can speak authoritatively. And according to Carnegie, the best speeches are given by people who know a topic very well.
"Talk about something that you know and know that you know," Carnegie wrote. "Don't spend ten minutes or ten hours preparing a talk: Spend ten weeks or ten months. Better still, spend ten years."
Making a speech personal is the shortest route to winning people over, according to Carnegie.
"Carry a sheet of paper with you for a few weeks and write down, as you think of them, all the subjects that you are prepared to talk about through experience," Carnegie suggested.
These topics can include a big regret you have, your biggest ambition, or why you liked or disliked school.
If you watch Buffett speak, you'll see that he rarely looks at a piece of paper, if ever. That's because one of Carnegie's core principles is that a good speech is never typed up beforehand.
Instead, Carnegie recommended referring to brief notes, rather than reading from a transcript that can keep you from being present.
"When you stand up to talk, you will probably find yourself trying to remember what you wrote," he wrote. "That will keep you from speaking naturally and with sparkle."
Smiling, having "positive energy" and exuding confidence make a huge difference, Hart says.
Buffett became passionate about investing, money and achieving success at a young age. That enduring enthusiasm comes through in his speeches and interviews.
As Carnegie put it, "Even people with only mediocre speaking ability may make superb talks if they will speak about something that has deeply stirred them."
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This is an updated version of a story that appeared previously.