The recent stock market plunge has whiplashed many investors and left them wondering what to do about their portfolios.

But in times like these, don't panic. Instead, you want to think about the three D's, says Erik Davidson, a chartered financial analyst and senior director of investments at Wells Fargo Private Bank in Denver.

The first D — employing a defensive strategy — could lead you to make a mistake, Davidson said. "The big question out there from investors is, 'Is it time to get defensive with my portfolio?' " he said.

And by that they mean: Should they sell off everything and head for safer ground such as certificates of deposit, treasury bonds or money market accounts?

However, Davidson advises that in the current market environment, the two other D's are more important — being diversified and disciplined.

Let's look at that second D — diversification. If you are investing through your employer's retirement plan, such as a 401(k), go online to view your portfolio or get your last quarterly statement. Look at where and how you've allocated your money. Is it spread across various asset classes? Or have you concentrated your contributions in just one or two types of asset categories? In other words, is your money all in one basket?

Some hardheaded people who put all or most of their money in real estate or mortgage-backed securities are taking a well-deserved whipping right now. They ignored the nine most important words in investing: "Past performance is not a guarantee of future results," Davidson said.

As investors have been reminded of late, what goes up can come crashing down — and fast.

If you have a diversified portfolio, you don't have to be fearful of current events, says Jeff Seely, chief executive of ShareBuilder, an online brokerage that caters to small investors and is designed for long-term investing.

If you select good fundamental stocks, or broad exchange-traded funds, those investments should produce gains longer term, and a downturn is a time to buy, he said.

"Market normalcy is in fact a sequence of ups and downs, but with a general up-trend over time," Seely said. "Taking the dips along with the upticks is unfortunately an inescapable part of the process."

Davidson said that if you want to avoid reaching for antacid during times like this, divide your investment contributions so that a certain percentage is invested in a variety of assets, such as large-cap or large, blue-chip stocks. You should have some money in mid-cap and small-cap stocks (medium- and small-sized companies). Put a percentage in real estate and international securities, and in fixed-income securities, such as bonds.

How much you put into each category depends on how much risk you can stand. If you're investing in an employer-sponsored retirement plan and you're unsure how to allocate your contributions, contact the firm that manages the plan. Often it can help you develop an asset-allocation plan based on your risk tolerance and your investment time horizon.

Once you develop a plan or asset-allocation approach, you've got to be disciplined and stick to it. If you already had a plan, dust it off.

Let's say you decided you only wanted to invest 20 percent of your 401(k) contributions in a real estate investment trust, which is a security that sells like a stock and invests either directly in properties or in real estate-related loans, such as mortgages. During the run-up in the real estate market, your investment may have grown so that it represents 40 percent of your portfolio. Although it's great that it did so well, at some point you may want to rebalance your portfolio.

If you want to avoid making investment decisions based on your emotions, consider using a dollar-cost averaging investment strategy, Seely says. An investor using this method invests the same dollar amount at regular intervals regardless of a fund or stock's price as a way to ride out volatility.