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.

And 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 (73% total average returns versus 29% in the S&P 500). So it can be done.

You, too, can construct your portfolio in a sensible enough way that you 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 you've bet big on the success of satellite radio and own both Sirius Satellite Radio (NASDAQ:SIRI) and XM Satellite Radio (NASDAQ:XMSR), would it make sense to buy an equipment maker whose revenues depend on that industry? Probably not, because you have too much riding on the sector's market-beating success.

The next logical step, then, is to aim for great stocks with a low or negative correlation (click here for more information on this fascinating topic). That simply means that although they'll all hopefully rise in value over the long term, the stocks will tend to move in relatively different directions along the way. One zigs, the other zags, and 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 notes that "anything less than perfect positive correlation can potentially reduce risk."

The right way to do it
Here's a good example. In Stock Advisor, Tom refers to his method as "industry rotation." His goal is to find the best companies in beaten-down industries 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 Schlumberger (NYSE:SLB) and ExxonMobil (NYSE:XOM) 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 like Corporate Executive Board have more than tripled in value since. He then moved on to health care, recommending companies in that industry several times in 2003. The average total return of those 10 stocks is 153% -- versus 33% for identical amounts invested in the S&P 500. After that, he liked the potential in tech stocks, particularly the semiconductor sector. Again he found success. Nowadays, he's looking for bargains in the home entertainment industry.

Let's take a look at two other Stock Advisor picks from different industries to highlight the diversification benefit. UnitedHealthGroup (NYSE:UNH) supplies health-care services to individuals. SiliconLabs (NASDAQ:SLAB) designs integrated circuits for consumer electronics items. Both have moved differently enough so that their combined returns were much smoother than either of them achieved separately. In the past six months, Silicon Labs has suffered a drop, while UnitedHealth has achieved closer-to-market returns.

The Foolish bottom line
I don't use these examples to applaud Tom and David's stock-picking skills. Rather, my aim is to highlight the importance of diversification, even when choosing a relatively small number of stocks. Certainly, 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 get you not only every recommendation they've ever made, but you'll also be able to follow Tom along the industry carousel. Plus, each newsletter now includes Tom or 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. At the time of publication, he owned no companies mentioned in this article. UnitedHealth is also an Inside Value selection. XM Satellite is a Rule Breakers pick. The Motley Fool isinvestors writing for investors.