The easy trick that helped one 26-year-old save $18,432 in 6 months

One simple going-out rule saves this reporter $3,600 each year

If you're trying to save more money, how you approach the challenge might make all the difference.

For some, that means cutting out small, daily expenses, such as a latte or avocado toast. For others, it means going cold turkey on spending for an entire year. And for one millennial, it means turning saving into a game.

David of Zero Day Finance, who goes by only his first name online, uses a simple, relatively moderate strategy to minimize his spending. The 26-year-old New Yorker commits to at least one "zero spend" day a week during which he actively avoids buying anything, including a morning coffee or an item from the drug store.

David tracks his progress with the challenge on his blog, where he "collects" zero spend days and pushes himself to fit as many of them as possible into a week. By gamifying his spending, he stays motivated to save.

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"As a competitive person, I always want to improve my numbers," he writes on his blog. "This means the better I am with the challenge, the less money I spend."

Since starting the challenge six months ago, David has saved $18,432, cutting his monthly spending from around $4,700 a month to $3,170 a month. That's a 33 percent decrease and saves him enough to max out his 401(k).

Not only does David now have more room to invest in retirement savings, he's changed his entire financial mindset. "At first, spending less money on 'stuff' made me a little depressed. I was so attached to spending money that it affected my emotional state," he writes. "But as the days and then weeks went by, something amazing happened. I started going outside and found ways to have fun on my own. I stopped relying on spending money to give me happiness, and started drawing it from creating things."

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While he still faces temptations to spend, he now runs every purchase through a mental filter. Is buying X item going to make his life better, or is it just stuff that's going to pile up?

"I no longer find myself swiping my credit card in an attempt to solve my problems and unhappiness," he says.

With a challenge like this, the percentage everyone will be able to save will be different, of course. But even increasing your retirement savings by one percent each month can make a major difference thanks to the power of compound interest, in which any interest earned begins to accrue interest itself. So a little money invested today shakes out to more than a lot of money invested later.

If you want to pay off debt, retire early or finish rich, start saving now, no matter if it's $100 a month or $1,000.

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This simple going-out rule saves this reporter $3,600 each year