Berkshire Hathaway CEO and self-made billionaire Warren Buffett turned 89 on Friday, August 30. He's also celebrating his 13th wedding anniversary with his wife, Astrid. In honor of the Oracle of Omaha's big day, CNBC Make It rounded up seven of his best pieces of life advice.

Marry the right person

Buffett made his fortune through smart investing, but if you ask him about the most important decision he ever made, it would have nothing to do with money. The biggest decision of your life, Buffett says, is who you choose to marry. "You want to associate with people who are the kind of person you'd like to be. You'll move in that direction," he said during a 2017 conversation with Bill Gates. "And the most important person by far in that respect is your spouse. I can't overemphasize how important that is." It's advice he's been giving for years. As he said at the 2009 Berkshire Hathaway annual meeting: "Marry the right person. I'm serious about that. It will make more difference in your life. It will change your aspirations, all kinds of things."

Warren Buffett and his wife Astrid Buffett arrive to a state dinner hosted by U.S. President Barack Obama and first lady Michelle Obam at the White House in Washington, D.C., U.S., on Wednesday, March 14, 2012. Andrew Harrer | Getty Images

Invest in yourself

"By far the best investment you can make is in yourself," Buffett told Yahoo Finance editor-in-chief Andy Serwer earlier this year. First, "learn to communicate better both in writing and in person." Honing that skill can increase your value by at least 50%, he said in a Facebook video posted in 2018.

Next, take care of your body and mind — especially when you're young. "If I gave you a car, and it'd be the only car you get the rest of your life, you would take care of it like you can't believe. Any scratch, you'd fix that moment, you'd read the owner's manual, you'd keep a garage and do all these things," he said. "You get exactly one mind and one body in this world, and you can't start taking care of it when you're 50. By that time, you'll rust it out if you haven't done anything."

By far the best investment you can make is in yourself. Warren Buffett Berkshire Hathaway CEO

Associate yourself with 'high-grade people'

Who you associate with matters, Buffett told author Gillian Zoe Segal in an interview for her 2015 book, "Getting There: A Book of Mentors." "One of the best things you can do in life is to surround yourself with people who are better than you are," he said. If you're around what he calls "high-grade people," you'll start acting more like them. Conversely, "If you hang around with people who behave worse than you, pretty soon you'll start being pulled in that direction. That's just the way it seems to work."

Work for people you respect

"Try to work for whomever you admire most," Buffett told Segal. "It won't necessarily be the job that you'll have 10 years later, but you'll have the opportunity to pick up so much as you go along." While salary is an important factor when thinking about your career, "You don't want to take a job just for the money," said Buffett. He once accepted a job with his mentor and hero, Benjamin Graham, without even asking about the salary. "I found that out at the end of the month when I got my paycheck," he said.

Benjamin Graham in 1955 Bettmann | Getty Images

Ignore the noise

Investing can get emotional, and it doesn't help that you can see how you're doing throughout the day by checking a stock ticker or turning on the news. But no one can be certain which way the financial markets are going to move. The best strategy, even when the market seems to be tanking, is to keep a level head and stay the course, Buffett says. "I don't pay any attention to what economists say, frankly," he said in 2016. "If you look at the whole history of [economists], they don't make a lot of money buying and selling stocks, but people who buy and sell stocks listen to them. I have a little trouble with that."

Success isn't measured by money