Steve Siebold has spent the last 30-plus years interviewing over 1,300 self-made millionaires and billionaires. And during that time, he's noticed something they all seem to have in common: a similar mindset.

In fact, Siebold says mindset is what sets apart successful people from those who are unsuccessful.

But Siebold hasn't just studied the mega-rich (for his books "How Rich People Think" and "177 Mental Toughness Secrets of the World Class") — he's also a self-made millionaire. He's a former professional tennis player and national coach, and began working with Fortune 500 sales teams in 1997, helping behemoths like Johnson & Johnson and TransAmerica ramp up their sales. As a professional speaker, his website states he's ranked among the top 1 percent of income earners worldwide.

And according to Siebold, the magic starts to happen in your head.

People have one of two mindsets: scarcity thinking and world-class thinking, Siebold explains.

Too many people operate "through fear and scarcity," says Siebold.

But successful people don't think that way, he says.

"The world-class operates through more of a love and abundance mentality, so they're seeing the opportunities," Siebold explains to CNBC Make It.

"They're looking for the opportunities, they believe they can succeed in the opportunities, where the rest of us are sort of in that fear and scarcity mindset and we're trying to survive."

Virgin Group founder Richard Branson's willingness to try things is a good example. The billionaire previously revealed that he's a big proponent of saying "yes." Even though he admits that he's probably said yes too many times in his life, Branson says he has no regrets.

"Even if I have no idea where I'm going or how to get there, I prefer to say yes, instead of no," he writes on his blog. "Opportunity favours the bold.

"If somebody offers you an amazing opportunity but you are not sure you can do it, say yes — then learn how to do it later!" he says.

The same kind of can-do thinking also saved the iPhone.

Just months before Apple was to release the first iteration of the smartphone in 2007, co-founder Steve Jobs pivoted. He decided he wanted it to have a glass screen rather than a plastic one that would get scratched. Jobs gave the manufacturer of the glass a tight deadline, and when the manufacturer said it wasn't set up to mass produce the product yet, Jobs urged him to think differently.

"Don't be afraid," Jobs reportedly said. "You can do it. Get your mind around it. You can do it."

The supplier met the aggressive deadline.

The difference in thinking is "like the tale of two different cities or worlds," says Siebold. "People living right next to each other, living in completely different worlds psychologically."

Luckily it's possible to develop the "world-class" mentality yourself.

First, advises Siebold, who you associate yourself with can play a big role in adopting the mindset of a world-class thinker. He explains that super successful people often hang around each other, and tend to live in the same neighborhoods or go to the same clubs.

"Because of that association, they continue to raise up their thinking," Siebold says. "They keep raising their level of thinking, while the average person keeps associating with the average person and they keep re-enforcing that low level consciousness."

"Limit your associations with people who aren't succeeding, and start increasing your associations with people who are," he advises.

It's not hard to do these days, even if you don't live within the gates of a posh community or rub elbows with the 1 percent, Siebold says.

Get exposure to the type of thinking that yields success, says Siebold, by studying "big thinkers" like Branson and Jobs, or Warren Buffett and Bill Gates, through reading.

"You could associate with very successful people just by watching their videos on YouTube," adds Siebold. In fact, CNBC has an entire website dedicated to footage of Buffett interviews and video recordings.

"You get exposure to the way they think, and you realize psychologically, they are literally psychologically living in a different world."

Because success "starts with your consciousness, the way you're thinking," Siebold reiterates. "That's the root of everything, from my experience."

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