Make no mistake -- investing can be risky. But it doesn't have to be too risky. Warren Buffett once said that rule No. 1 for investing is to "never lose money," with rule No. 2 being "never forget rule No. 1."

Even Buffett hasn't been able to totally adhere to those rules in his investing career. There are some stocks that improve your chances of doing so, though. If you're a risk-averse investor, Buffett's own Berkshire Hathaway (NYSE:BRK-A) (NYSE:BRK-B), along with Iron Mountain (NYSE:IRM) and Waste Management (NYSE:WM), are top stocks to make solid returns over the long run.

Hands holding caliper measuring the word "risk"

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Berkshire Hathaway

Berkshire Hathaway's risk advantage stems from its diversification. It's not really just one company. Instead, more than 60 different companies fall under Berkshire's umbrella. These include insurers such as Geico and GenRe, retailers such as Borsheims Fine Jewelry and See's Candies, and manufacturers such as Benjamin Moore and Precision Castparts, in addition to companies in several other industries.

Those are just the companies that are part of Berkshire. It also owns stakes in more than 40 publicly traded companies across multiple industries.

Warren Buffett's strategy has certainly worked in the past for Berkshire Hathaway. The stock has generated compound annual returns of nearly 21% since 1965 -- more than doubling the return of the S&P 500 index during that period. Those gains aren't just a thing of the past: Berkshire stock has enjoyed a terrific year in 2017, with its share price up almost 21% year to date.

Will Berkshire produce market-beating returns year in and year out? Probably not. Over the long run, though, the company's diversification and shrewd investing strategy should help investors abide by Buffett's top two rules more often than not.

Iron Mountain

Iron Mountain has a different kind of diversification. The records and data storage company counts more than 230,000 organizations as customers. That list includes around 95% of the Fortune 1,000.

Aside from this huge customer base that continues to grow, Iron Mountain's business itself helps reduce risk for investors. The company provides services that will only increase in demand. Organizations continue to generate more records and data than ever before. They must retain these records and data for legal reasons. As a dominant player in the records and data storage market, Iron Mountain is often the first provider these customers turn to.

When customers pick Iron Mountain, they tend to remain customers for quite a while. Half of the boxes stored in Iron Mountain's facilities have been there for 15 years. One-quarter of the boxes have been there for 22 years. That excellent customer retention stems from the simple fact that it's too much of a hassle for most customers to move their records somewhere else. 

There's also one other reason Iron Mountain is less risky than most stocks: its dividend. As a real estate investment trust (REIT), the company must return at least 90% of its earnings to shareholders in the form of dividends. Iron Mountain's dividend yield currently stands at 5.8%, giving shareholders a nice cushion even if the stock declines. No cushion has been needed in 2017, though. Iron Mountain stock is up 25% so far this year. 

Waste Management

In a way, Waste Management's low risk level comes from the exact opposite reason than that of Iron Mountain. Instead of succeeding because customers need to hold on to records, Waste Management thrives because they need to discard them -- as well as lots of other things.

Waste Management is the leading provider of comprehensive waste management environmental services in North America. The company owns or operates 248 landfill sites and manages 310 transfer stations that consolidate, compact, and transport waste. 

While Waste Management's business might not be the most glamorous, it's certainly lucrative. The company is on track to generate revenue of around $14.4 billion in 2017, with earnings of close to $1.4 billion. In the third quarter, Waste Management's management said its cash flow reached all-time high levels

More good news for investors is that Waste Management like to return some of that impressive cash flow to its shareholders. The company's dividend currently yields north of 2%. Future dividend increases seem likely.

Some risks remain

It should be noted that lower risk doesn't mean no risk. All three of these companies face some threats that could derail their stocks. Deteriorating economic conditions in the U.S. could cause problems for each of the stocks. Because Wall Street places a heavy emphasis on quarterly results, a significant earnings miss could make any of these stocks drop.

Those risks are temporary in nature, though. Berkshire Hathaway, Iron Mountain, and Waste Management have solid business models and strong financial positions that should make them great -- and relatively low risk -- picks over the long run.

Keith Speights has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends Berkshire Hathaway (B shares). The Motley Fool has a disclosure policy.