NEW YORK (CNN/Money) - Diversify, diversify, diversify...most investors have had this investing mantra drilled into their brains by now.

But even if they've been good about not putting too many eggs in any one stock, one sector, or one asset class, many U.S. investors are still relying too much on one country.

Remember, a truly diversified portfolio means a global portfolio -- and that's true now more than ever.

The world is increasingly becoming one giant marketplace for goods and services.

China, for example, is now a key global manufacturing center, making everything from computer parts to sneakers. And with 1.3 billion citizens, that build out creates an insatiable need for energy, steel, lumber and shipping, to name just a few things.

India has become an integral world technology center, with one of the fastest growing economies. As those workers there rise through the economic ranks, they're buying homes and cars, and signing up for credit cards.

Brazil has become the world's No. 1 food supplier; Russia is sitting on billion of barrels of untapped oil.

If these trends alone don't grab you, consider the returns behind them. An S&P 500 index fund has yielded barely 2 percent so far this year, while the ING Russia fund (LETRX) is up 52 percent and the Eaton Vance Greater India fund (EMGIX)is up 31 percent.

It's always dangerous to chase after whatever market is hot, but many overseas stocks are still looking pretty cheap.

Russell Lundeberg, chief investment officer at Barrett Capital Management in Richmond, Va., says European stocks are trading at a 30 percent discount to U.S. equities. The average dividend is greater, too, at 2.8 percent versus 1.8 percent.

To get started, you first need to figure out how much you want to invest. A well rounded portfolio should include 10 percent to 30 percent of non-U.S. holdings. These assets can include stocks or bonds of emerging or developed non-U.S. markets, depending on how much risk you want to take on.

Emerging markets have the strongest growth prospects, but these markets are also the most volatile.

Political instability, currency fluctuations and corporate scandal are just a few things that can tank stocks in such market.

And remember that these are still small markets: The value of all the shares on China's two primary exchanges total $450 billion, less than the value of just two U.S. giants, Exxon Mobil and General Electric.

What's more, getting reliable information can be tough -- Mark Kajita, vice president with Baker Boyer National Bank in Walla Walla Washington, estimates that 70 percent of stock in China is owned by the Chinese government.

If you don't have the stomach or long term outlook for serious market swings, it's best to focus on developed countries like Japan or Western Europe. And since buying shares in international markets is tough, most investors are better off sticking with mutual funds.

Christine Benz, associate director of fund analysis at Morningstar, in Chicago, recommends a diversified foreign stock fund, like Artisan Interantional (ARTIX) or American Funds EuroPacific Growth Fund (AEPGX), rather than a fund focused on a specific region.

"These are both sturdy, core funds, they're relatively cheap and they have a fair amount of analysts who are overseas," she said.

One important thing to keep in mind when choosing funds: The terms "international," "worldwide," "global" and "foreign" may sound similar, but they're not.

Funds that use the first three terms in their name may have up to 50 percent of assets invested in U.S. companies.

Foreign funds usually do have steeper management fees, but many actively managed funds outperform their benchmarks, making the costs worthwhile.

"The less efficient the market, the more useful a good fund manager is," said Lundeberg. "Most actively managed U.S. firms underperform the market, but the opposite is true in emerging markets for good managers."

If the high fees associated with international stock funds turn you off, exchange traded funds, or ETFs, are a lower cost option. ETFs resemble index tracking funds, but they're bought and sold like stocks. That's another advantage -- ETFs trade throughout the day on an exchange, while mutual funds prices are set just once a day.

The downside: If you're making many small purchases, you'll pay a commission -- as with stocks -- each time. Good information on ETFs can be found at etfconnect.com, etfzone.com, and ishares.com.

Finally, before you start buying, it's important to make sure you know where you've already put your money. Many U.S. companies, like McDonalds, Starbucks, Procter and Gamble and Avon all have a large amount of overseas exposure, and that's where they're seeing their strongest growth today.

So the stocks and mutual funds you already own may already be working overseas for you, you just don't realize it.

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Carrie Lee is a morning business anchor for CNN Headline News.