Chances are you weren't happy with your investment returns this decade. That's because the 10 years ended 2009 were the worst-performing decade in the history of the stock market. Stocks in the aughts averaged an inflation-adjusted 3.3% annual decline.

That's bad, but there is a bright side. If you know your stock market history, then you know that just as periods of outperformance are often followed by periods of underperformance (the 1990s was one of the market's best decades), periods of underperformance are often followed by periods of outperformance.

In other words, the next 10 years will likely be far better than the last.

Don't blow this opportunity
That tailwind will benefit our returns for the next 10 years. And if we play our cards right by taking a long-term view, regularly adding new money to the market, minimizing taxes and transaction costs, reinvesting dividends, and focusing on superior companies, we should be able to look back at the end of 2019 and say that we had a pretty good decade, investing-wise.

But -- and this is important -- if you want to make the next 10 years one of the best investing decades of your life, you might heed the advice of one of the world's top investors and consider a minor strategic shift.

Did you know ...
Before I can talk about that shift, I need to ask: Do you remember Oct. 29?

It was a joyous day! The stock market rallied on the news that U.S. GDP had grown 3.5% in the third quarter -- news that ostensibly marked the end of our 18-month-long recession.

Yet it turns out that the news wasn't as good as originally broadcast. The Commerce Department later revised its third-quarter GDP growth estimate down to 2.2%. Net out the 1.66% bump the number got from the government's ill-conceived and unsustainable Cash for Clunkers program, and it now looks like our economy in the third quarter really didn't grow that much at all.

Are you too U.S.-centric?
Let's tie these two stories together. Why did U.S. investors like us suffer such disappointing returns this past decade? In short, we didn't adjust our investment philosophies to account for a dramatically different world.

U.S. economic growth has slowed, and as the revisions to the third-quarter GDP growth number reveal, it shows limited signs of life. At the same time, however, India and China put up third-quarter GDP growth of 7.9% and 8.9%, respectively. Yes, those economies also benefited from public-sector stimulus, but the difference between their numbers and our own is so stark that one can't help but acknowledge their rosier forward economic outlooks.

Fast-forward a few years, and this will lead to what PIMCO co-chief investment officer Mohammed El-Erian calls a "multipolar world" -- one that is no longer reliant on the U.S. economy to drive growth. This, he told Fortune, is no doubt good for the globe. "Most of us would rather be on a plane with multiple engines," he pointed out.

Yet if you, as an American investor, continue to focus on American stocks -- and American investors are far too exposed to American stocks -- your investment returns over the next decade may end up being as disappointing as those of the past decade. This, El-Erian points out, is the consequence of being "too U.S.-centric in a globalizing world where the center of gravity is shifting."

Do something about it
This is a real and significant risk that all American investors are facing today, but this is where the aforementioned minor strategic shift comes in. El-Erian says that all you have to do is "recognize that the asset allocation of tomorrow is much more global than the asset allocation of yesterday."

Or in plain English: Set aside some time and money this year to buy more foreign stocks.

I know, I know ...
Despite the surging popularity of emerging-market investments, many Americans remain hesitant to own foreign stocks. That's because they don't know these companies and may not feel comfortable evaluating their financial statements or market opportunities.

That's an understandable objection -- but it still shouldn't lead you to blow this opportunity. So let me give you a head start by telling you a few of the stocks we consider a buy at our Motley Fool Global Gains international research service.

Get your pad and paper ready
One is Philip Morris International (NYSE: PM), the world's largest cigarette company. It's not only owner of one of the world's biggest brands (Marlboro) and a multi-tiered product portfolio, but it's a cash flow machine with a demonstrated history of rewarding shareholders through dividends and share repurchases. Even better, its price has been trending down of late due to concerns about the euro and recent larger than expected sales volume declines. Put simply, this is a higher quality company and less expensive than competitors in the space such as British American Tobacco (NYSE: BTI).

Another name we like is Unilever (NYSE: UL), a UK-based global consumer products company that competes with the likes of Procter & Gamble (NYSE: PG), Campbell Soup (NYSE: CPB), and Heinz (NYSE: HNZ) -- though it offers more foreign-market and currency exposure than these U.S.-based counterparts. The stock is cheap, the balance sheet is clean, and the company is doing a great job positioning itself relative to the competition in fast-growing markets in Asia, Africa, and Eastern Europe.

But once you buy Philip Morris and Unilever, you won't want to stop there. Your portfolio needs exposure to international markets not only through multinationals, but also through companies based in important emerging markets as well. So if you'd like further insights into our top picks from around the world, simply sign up to get all of our Global Gains research free for 30 days. Click here to get started -- there's no obligation to subscribe.

This article was first published on Jan. 29, 2010. It has been updated.

Tim Hanson is co-advisor of Motley Fool Global Gains. He owns shares of Philip Morris International. Philip Morris International and Unilever are Motley Fool Global Gains selections. H.J. Heinz, Procter & Gamble, and Unilever are Income Investor selections. The Fool owns shares of Procter & Gamble. The Fool has a global disclosure policy.