One thing that often gets lost in all the talk of sizzling stocks and 20-baggers is the benefit of diversification. It's a concept every investor can understand and profit from.

Don't think diversification automatically means mediocre returns. David and Tom Gardner have led Motley Fool Stock Advisor members through a variety of industries, and even some international exposure, on the way to outsized performance -- 64% total average returns, versus 26% in the S&P 500. So it can be done.

You, too, can sensibly construct your portfolio to significantly lower your risk, even if you own very few stocks. Here's how.

Negative is good
Always try to consider how each potential new purchase relates to the rest of your portfolio. If Wal-Mart (NYSE:WMT) and Target (NYSE:TGT) already make up a big chunk of your holdings, it's best to avoid another discount retailer when buying more stock. Even if you think it would provide you with great returns, if the industry falls apart (translation: you ended up being wrong), you'd be in for a lot of pain. Even if you ended up being right in your analysis, you'd still be in pain if you had to sell unexpectedly and the industry remained depressed for an extended period.

The next logical step, then, is to aim for great stocks with a low or negative correlation. That simply means that although they'll all hopefully increase in value over the long term, they will tend to move in relatively different directions along the way. One zigs, the other zags, but it's OK: Your portfolio's returns will be much smoother as a result.

The best news is that it's fairly easy to gain this benefit. In his classic book, A Random Walk Down Wall Street, Princeton professor Burton Malkiel says, "Anything less than perfect positive correlation can potentially reduce risk." He's right. Even archrivals Dell (NASDAQ:DELL) and Hewlett-Packard have different enough business models to provide some low correlation.

The right way to do it
In Stock Advisor, Tom refers to his method as "industry rotation." His goal is to look at beaten-down industries and find the best companies that are ready to rebound. Such stocks are among the market's best performers when the turnaround comes, as it inevitably does in a relevant industry. Think of how Chevron (NYSE:CVX) and Total SA (NYSE:TOT) rocketed ahead when the oil industry recovered -- each easily topping the market over the past couple of years.

When the Stock Advisor service began in early 2002, Tom saw value in the financial and business services sector, and picks such as Corporate Executive Board have more than doubled in value since. He then moved on to health care, recommending companies in that industry several times in 2003, including Laboratory Corp. of America, which has doubled. After that, Tom liked the potential in tech stocks, particularly the semiconductor sector. Again he found success. Nowadays, he's looking for bargains in other industries.

I don't use these examples to applaud Tom and David's stock-picking skills. Not every recommendation is a winner. For example, September 2003 pick Possis Medical (NASDAQ:POSS), which Tom later recommended selling, is down 35%.

Instead, my goal is to reiterate the importance of diversification, even when choosing a relatively small number of stocks. Not all stocks will be winners, but by spreading picks across sectors, you're giving yourself a chance for sensible diversification -- and a smoother road to long-term success.

Right now, a 30-day free trial to Stock Advisor will let you see every recommendation the Gardner boys have ever made. Plus, each newsletter now includes Tom and Dave's top five stocks to buy now. Here's more information.

This article was originally published as "Are You Invested in the Right Industries?" on April 5, 2006. It has been updated.

Rex Moore never met a deadline he didn't meet. He owns no companies mentioned in this article. Dell is a Stock Advisor and Inside Value recommendation. Wal-Mart is an Inside Value recommendation. Total SA is a Motley Fool Income Investor pick. The Motley Fool is investors writing for investors.