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One of the biggest challenges of retirement is saving for it. But another challenge is that you only do it once.

When you've done something once, you have an idea of how to do it again for example, once you've bought your first home, buying your next home feels a lot less intimidating. But retirement isn't like that it's not something you have another shot at later on.

Luckily, lots of people have retired comfortably and are willing to share the steps they took to do so. Here, we've compiled the best advice from four retired people for anyone who's looking forward to a long and happy retirement.

Business Insider contributor Sean, of The Money Wizard, didn't find out that his grandfather was a millionaire until he was comfortably retired.

For years when Sean was growing up, his grandfather drank the cheapest beer, pinched pennies, and drove older-model cars. His grandfather brought in an average salary and never earned a college degree while raising five children. But eventually, Sean found out his grandfather had a retirement account worth $1.2 million.

"My grandpa eventually let me in on his secret: Wealth doesn't mean earning a lot of money. Nor does it mean spending a lot of money. Instead, wealth is all about investing inincome-producing assets,"Sean writes.

One of the biggest factors in building wealth through investing is how long investments can stay in the market. He learned early that investing consistantly and earning average stock market returns could help him grow his savings over his long career.

"He only needed to invest about $8,000 per year for his portfolio to grow from nothing to $1.2 million," Sean writes. And he focused on buying assets and investments instead of things.

Sean says that his grandfather's advice has helped him get on track to hit $6.8 million at age 60, or retire early.

Writer Michelle Jackson's grandmother only started saving for retirement 10 years before she retired. Jackson's conversations with her grandmother confirmed that it's never too late to start saving.

Jackson's grandmother started saving 10% to 15% of her paychecks in her 50s. And it all worked out she retired comfortably in her 60s. And she's happy, Jackson writes. "In observing my grandmother's financial habits, I found myself realizing that she feels quite wealthy. She has enough. Enough money for the things she enjoys."

Like Sean of The Money Wizard, Business Insider contributor Eric Rosenberg didn't know his grandfather was a millionaire until later in his life. Rosenberg's grandfather using a simple buy-and-hold investment strategy to reach millionaire status, while similarly not flaunting it.

Buying and holding shares is exactly as it sounds: buying shares of stock and keeping them for years. While it isn't the most glamorous way to invest, Rosenberg's grandfather is proof that it works. "Picking good companies and sticking with them can pay off very well in the long run. He didn't follow some get-rich-quick scheme," Rosenberg writes. "He picked solid companies and held on as they grew in value over time."

While Rosenberg's grandfather built his portfolio by buying into Walmart very early on, it's also possible to use ETFs to invest in multiple companies at once and see growth. "Buying a low-cost S&P 500 ETF and investing more steadily over time gives you investment exposure to 500 of the biggest stocks in the United States. If history continues to repeat itself, chances are good you would do well with that type of investment over time," he writes. Index funds can also be used, which track individual industries the way S&P 500 tracks the largest 500 companies.

Buy-and-hold investing doesn't require much maintenance when stocks simply sit and grow over many years. It's a relatively simple strategy that's a great option for anyone who still has many years before retirement.

For many people, retirement is lasting longer. Personally, I learned about the benefits of long-term care insurance, a type of insurance that can help pay for assisted living and in-home care later in life, from my grandmother, as I've previously written.

When my parents could no longer care for my grandmother full-time, my grandma's experience moving into an assisted living facility taught me just how important long-term care insurance is, especially for women who tend to live longer. Typically, this type of insurance is most affordable to purchase between ages 52 and 64.

Now in her 90s, my grandma had a long and comfortable retirement before moving into the facility. But, having that coverage helped to make sure she could afford to do so, and continue to live comfortably in her later years.

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4 pieces of advice to help you retire comfortably, according to people who have done it - Business Insider