39-year-old retired millionaire: The best investment I ever made cost 'little to nothing'

How this average 38-year-old became a millionaire and retired early
How this average 38-year-old became a millionaire and retired early

Chris Reining, 39, managed to build a seven-figure portfolio by age 35 and quit his corporate job shortly after. He did it by saving more than half his income and investing it in his employer sponsored 401(k) plan and a taxable account.

Needless to say, Reining, who has been retired for nearly three years and is "not really worried about ever running out of money," made some smart investments over his short career.

The very best investment he ever made, though, has nothing to do with the stock market or real estate or money in general: He invested in himself, he tells CNBC Make It, and that cost "little to nothing."

Investing in yourself can mean different things. For Reining, it meant getting mentors to guide him; surrounding himself with smart, driven people; reading self-help books; and practicing new skills like public speaking by joining Toastmasters.

"One of the best shortcuts to wealth is making yourself more valuable," the self-made millionaire writes on his blog. And, "when you invest in yourself you become more valuable."

One of the best shortcuts to wealth is making yourself more valuable.
Chris Reining
self-made millionaire

Self-made millionaire Grant Cardone, who was deep in debt before building five businesses and a multimillion-dollar fortune, agrees. "The best investment you will ever make is in yourself," he says.

That's advice that his mom gave him. "It's a-no lose deal," she told Cardone, he recalls. "It will always give you a return. Nobody can take it from you. It's yours.'"

He put his mom's advice into effect when he was 25 years old and struggling to make ends meet with his job at a car dealership. "Even though I hated the job, I decided I would throw myself into my sales job 100 percent," he writes of his younger self in "Be Obsessed Or Be Average."

Cardone watched sales training videos while he ate breakfast, listened to self-improvement tapes during his drive to work and was often the first employee to show up and the last to leave.

He started to see results: He went from making $3,000 a month to $6,000 a month in his commission-based job. By 30, the entrepreneur had notched his first million.

How Warren Buffett overcame his crippling fear of public speaking
How Warren Buffett overcame his fear of public speaking

Even legendary investor Warren Buffett says that investing in yourself is the one investment that "supersedes all others." He told Forbes, "Nobody can take away what you've got in yourself, and everybody has potential they haven't used yet."

Acknowledge your weaknesses and commit to starting to improve them. "Don't put it off to your old age," he says. "Whatever you want to learn more, start doing it today."

If you do so, "you'll have a more rewarding life not only in terms of how much money you make," says Buffett, "but how much fun you have out of life; you'll make more friends the more interesting person you are. So go to it, invest in yourself."

Don't miss: Self-made millionaire says the mindset he adopted in his 20s helped him retire in his 30s

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This is Grant Cardone's number one tip for success
This is Grant Cardone's number one tip for success