A millennial who became a millionaire after the 2008 crash says building wealth is about more than opportunistic investing. You also have to make lifestyle changes and load up on side hustles.

• Grant Sabatier, 38, became a self-made millionaire in five years.
• He hustled, cut expenses, and benefitted from one of the strongest bull markets in history.
• Sabatier says that economic uncertainty has created an "incredible opportunity to invest," but that there's more to building wealth than timing the market.

In 2010, Grant Sabatier had less than $5 to his name. But just five years later, at age 30, he had accumulated a nest egg of $1.25 million which allowed him to retire early — and go on to author the book "Financial Freedom", the blog "Millennial Money", and co-found BankBonus.com.

Sabatier, who is now 38 years old, said that he saw his wealth grow in part because he was lucky enough to invest during the stock market's surge over the past decade-plus.

"I was the beneficiary of one of the greatest bull markets in history," he told Insider.

But he says buying the dip at the right time isn't all it takes to build wealth, and that it should be supplemented with lifestyle changes and side hustles that increase your income. Sabatier says that he's seen many people "obsess about cutting back" on spending and make cuts that are "unsustainable and make them unhappy" — that's among the reasons he recommends people focus more of their energies on boosting their earnings.

"There's a limit to how much you can cut back, but not a limit to how much money you can make," he said. "I wish more people focused on the upside, rather than mitigating the downside."

Since fears of a recession spiked last year, some investors have begun preparing to capitalize on the next market downturn and become "recession millionaires." While the economy and the stock market don't always rise and fall in tandem, historically, a struggling economy has provided investors with opportunities to buy stocks on the cheap.

When Sabatier began investing in 2010, the US economy had already emerged from the Great Recession, and the stock market had recovered some of its losses. But the S&P 500 was still well below its 2007 peak, and Sabatier was able to ride the market wave over the next decade. From March 2009 to March 2020, the S&P 500 gained over 400% — the longest bull market run in US history.

While Sabatier recognizes the major role the stock market's surge played in his personal wealth building, he ultimately attributes his financial success to the steps he took to make additional money and cut expenses over a decade ago. That's because in order to have any chance at capitalizing on a down market and generating big returns down the road, he says one has to have the money to invest in the first place.

Sabatier says growing one's skills, negotiating a higher salary, starting a side business, and reading books about investing or entrepreneurship are good ways people can put themselves on a path to earning additional income.

In addition to his $50,000 digital-marketing job, Sabatier took on side hustles that included building websites, flipping domain names, pet-sitting, selling concert tickets, and flipping vintage mopeds. This helped him increase his income to well over six figures per year.

"If you want to reach financial independence as quickly as possible, you're going to need to up your side hustle game," Sabatier told Insider last year.

In his mid-20s, Sabatier said that he put roughly 80% of his earnings into index funds — and he recommends that young Americans invest "as much as you can now."

"You'll come out ahead over the next 10, 20, and 30-plus years," he said, adding that, "the best time to start investing was yesterday. The second best time is today."