A man who retired at age 30 shares the most important habit that helped him get there

Peter Adeney, better known as "Mr. Money Mustache" turned quite a few heads when he announced his retirement at age 30 back in 2005.

He wasn't inheriting a fortune, nor was he coming off a ridiculously high salary — rather, he'd been working as an engineer for 10 years, averaging a $67,000 annual salary.

He accumulated enough money to retire comfortably in Colorado with his wife and son, and even has enough savings to indulge in his guilty pleasure: "The tiny little kinds of imported cheese from that special area in the gourmet grocery store," he told Farnoosh Torabi on episode 38 of her podcast, "So Money."

He did it by cutting back on expenses and saving a lot: two-thirds of his and his wife's take-home pay, he explained to Torabi.

She asked Mr. Money Mustache — who has since built a cult following for his blog that teaches others to live a frugal, yet meaningful, life of leisure — to share his most imporant financial habit that helps keep his money safe and growing.

The 30-something retiree's secret boils down to the habit of thinking about money as something to invest rather than something to spend.

"The growing part of my money is pretty simple," he told Torabi. "I just like the idea of keeping all money invested. So if I run into a surplus sometime, I don't think of something to buy with it, I think, 'OK, I better get rid of this money and put it to work again.' So, I sweep it out of the bank account and into regular index funds."

While a raise, generous birthday gift, or lucky lottery winnings may trigger a shopping spree for most of us, Mr. Money Mustache's instinct is to invest surplus money, which in part eliminates any spending temptation that may arise.

So far, it's working for him.