No matter whether you're a college student looking to break into the workforce or well-established but still looking toward the next step in your career, there's one universal piece of advice that can help you be successful, writes John D. Spooner in his book "No One Ever Told Us That: Money And Life Lessons For Young Adults": Be confident in yourself and your abilities.
Spooner, a Boston Globe No. 1 best-selling author, was named one of 100 best financial advisors in America by financial-investment website Barron's.
In his book, he recalls a time when a young, self-employed barber named Karyn displayed this type of self-assurance. She was hanging up a shingle in an up-and-coming college town. To get the word out about her services, she scoured the colleges' shops and restaurants, introducing herself to the owners, salespeople, wait staff and managers.
"'I'll give them my card and ask for theirs,'" Spooner recalls Karyn told him. "'Occasionally, I'll hang out near classrooms where freshmen go to lectures.
"'I'll pass out my cards to them and chat. I give people introductory haircuts to show them how good I am. Then, they're hooked. They'll come into the shop and I've got [them] for years. Plus their friends.'"
It was this type of confidence and attitude that, according to Spooner, set Karyn apart from other stylists and helped her get ahead.
Some very successful leaders today also attribute their success in large part to their belief in themselves.
Billionaire entrepreneur Bill Gates says having confidence gave him the courage to pursue his passion for computers. Gary Vaynerchuk, founder of the multimillion-dollar digital marketing company VaynerMedia, says his confidence is what led him to success as well.
And Chrishon Lampley, the owner and founder of wine label Love Cork Screw, says her self-assurance is one reason her small wine brand is rapidly growing. Lampley recently appeared on the CNBC series "The Job Interview. "
If you want to develop your sense of self-confidence, hone your skills and work hard to improve, Spooner writes. The more you know about, and the better you are, at a certain skill, the more confidence you'll display when talking about or performing it.
There are some physical things you can do to portray confidence, too, say, if you're in a meeting or an interview.
Author and body-language expert Patti Wood says you should talk with your hands. "Power is communicated by gestures," she says.
And, according to career coach Becky Berry, you should make eye-contact with the person you're talking to, while trying not to fidget and sitting or standing upright. "When you hold or move your body the way you would like to feel," she says, the posture actually sends a message to your brain: 'Hey, I am feel great, positive and up.'"
Having confidence in yourself and your skills is "a concept you should take to heart," Spooner writes. "If you believe that you're good at what you do … then you can strut your stuff."
Like this story? Like CNBC Make It on Facebook!