When I look at Forbes' list of the world's billionaires -- people who make more in a day than I have in my entire lifetime -- it becomes quite clear that there are two ways to make stupidly large amounts of money:

  1. Build a company that grows to be worth billions.
  2. Become a successful investor.

Frankly, creating a billion-dollar business sounds hard. So let's ignore Bill Gates and look at Nos. 1 and 2 on the list, Warren Buffett and Carlos Slim.

A tale of two billionaires
Now, most people know Buffett. As a value investor, he walks right by the overhyped, exciting-looking investments, and heads straight for the boring, ugly, beaten-down stocks in the bargain bin. He buys newspapers. Paint manufacturers. Carpet companies. And, when he's feeling particularly frisky, he'll even buy an underwear maker.

Yet Buffett made himself billions. Go figure.

Mexico's Slim came in second on the Forbes list (published last March), but he's far less familiar than Bill and Warren. Between 2004 and 2006, Slim doubled his fortune. Between 2006 and 2008, he doubled it again. To make that money, surely Slim would have to be in some really exciting businesses?

Umm ... no. Slim has been in cigarettes, real estate, soda bottling, auto parts, and insurance. His bread and butter is South American telecoms like America Movil (NYSE:AMX), but he's also owned stodgy companies such as Altria and Kraft Foods (NYSE:KFT).

How they did it
So how the heck did these guys make so much money in such boring industries? By following two simple rules:

  1. Buy companies when they're cheap.
  2. Focus on excellent businesses.

Buffett made a fortune when he bought loads of American Express at incredibly low prices in the wake of the 1960s Salad Oil Scandal. He recognized that the company still had a solid brand and was dirt cheap. Over the years, Buffett has made a habit of buying great companies at good prices, including Coca-Cola (NYSE:KO), Iron Mountain (NYSE:IRM), and Wells Fargo (NYSE:WFC).

Slim's first huge opportunity came during Mexico's economic crisis in 1982. When international investors fled in panic, Slim noted that "the low value of many enterprises was even more irrational than the pessimism in the business community." He bought and reaped immense profits.

At this point, Bill Gates is looking kind of lonely, so let's go back to him. Everyone knows that Gates made his billions by founding Microsoft. But if you've never looked at his personal stock portfolio, the names would surprise you. For instance, Gates owns a stake in Republic Services (NYSE:RSG), a trash hauler, and PNM Resources (NYSE:PNM), an Albuquerque, N.M.-based energy holding company. At this point, it should be obvious: Bill Gates is into boring value stocks, too.

The Foolish bottom line
Now, there's a reason that three of the world's richest men are value investors. Each knows that value investing simply outperforms all other types of investing.

That's why it pays to stay on the lookout for value stocks, particularly when the market gets volatile. Buffett and Slim made some of their most successful investments by buying when everyone was panicking. You can, too.

If you're looking for assistance identifying the best opportunities, our Motley Fool Inside Value newsletter can help. Every month, we recommend the two best value stocks we can find. You can see all our recommendations -- including this month's top picks -- with a free pass here.

This article was originally published on April 2, 2007. It has been updated.

Fool contributor Richard Gibbons would make the list, if they would just expand it by another 500 million people. Richard and The Motley Fool both own shares of American Express. American Express, Coca Cola, and Microsoft are Motley Fool Inside Value recommendations. America Movil is a Global Gains selection. Kraft Foods and Republic Services Group are Income Investor picks. The Motley Fool has a disclosure policy.