With mutual funds, watch for hidden trading costs that aren't reflected in the expense ratio. Pore over the prospectus and ask the fund company for its "statement of additional information," where commission information may be detailed, said Christine Benz, the director of personal finance at investment research firm Morningstar.

"In general, a fund with a very high turnover rate that's trafficking in smaller-cap names is going to be the most vulnerable to hidden trading costs," she said.

3. Diversify

Diversifying a portfolio among uniquely performing assets is trickier as financial markets have become more complex.

"Investors now have access to many more strategies and investment styles than they did before," said Lo, the MIT professor. As a result, "many of these investments have become crowded trades."

Those crowds can foil diversification -- if investors pull out, it's a broad-based sell-off.

Investors now must work harder to find asset classes that diversify their portfolios while remaining within their risk tolerance. That includes looking at real estate, commodities and currencies. Still, Lo and others caution: Focus on what you understand.

Morningstar's Benz said she's not that enthusiastic about some alternative investments, such as funds that employ short-selling strategies. "A lot of alternatives are overpriced and will probably deliver returns that fall somewhere between stocks and bonds," Benz said.

Still, she said, commodities have a role in investors' portfolios. They can provide a hedge against inflation, but there are dangers, too.

Most commodities-focused exchange-traded funds, for instance, focus on the futures market so don't perfectly track commodity prices. "Some professional money managers seem to be veering away from commodities as an asset class and instead are talking about buying just natural-resources stocks or funds, such as T. Rowe Price New Era (PRNEX), for example," Benz said.

While alternative investments and tactical asset allocation are two strategies popular now to help investors weather storms, "I'm in favor of a more-traditional approach," Benz said. "If investors come up with a sane asset-allocation mix and adjust it gradually over time to make it more conservative, that is more than 90% of the battle."

Others agreed. "You can be extremely well diversified with five or 10 holdings," said Rick Miller, the founder of Sensible Financial Planning in Waltham, Mass. He suggested a portfolio with a total U.S. stock-market fund, international fund, emerging-markets fund, Treasury inflation-protected securities fund, total bond-market fund and REIT. "For people doing this themselves, that's great. It's cheap, and it's inexpensive to rebalance."

But, he said, diversification is about averaging out security-specific risk. "Market risk is undiversifiable. You cannot get rid of it unless you don't hold it."

Bergman made a similar point: "Diversification still works. It's just that when all hell breaks loose, correlations rise."

For the equity portion of his clients' portfolios, Bergman said he focuses on small- and large-cap stocks, international equities, natural resources and real-estate securities. In extreme periods, the price movement among those asset classes is high. "That's OK," he said. "We don't put any money that we need in the next five years into the stock market."

Evensky said his firm employs a "core and satellite" approach, with 80% of the portfolio invested to capture market-based returns cost-efficiently and the other 20% for risk taking -- assets that may be costlier and tax-inefficient but have potential returns to overcome those costs.

4. Rebalance

With market volatility as common these days as political gridlock, you may need to rebalance more often to stay true to your investment plan. That's good and bad.

Rebalancing provides an opportunity to harvest losses for tax purposes, plus it forces you to buy low and sell high. But moving money comes with transaction costs.

To reduce expenses, keep your portfolio simple. "If you have a broker or a custodian that has competitive expenses and you have an efficient portfolio (with just five or 10 holdings) then it's not going to cost you that much to rebalance," Miller said.

Some suggest rebalancing about once a year. Bergman of Palisades Hudson said he rebalances when an asset moves more than 10% beyond its target weight in a portfolio. That disciplined approach helps his clients weather bad times, and outperform in good times. In fall of 2007, before the downturn, "we were selling stocks," he said.

5. Be proactive, but patient

Sticking to your investing goals means having a strategic plan in place that you revisit periodically. "It's buy and manage," Evensky said, as opposed to buy and hold.

In times like these, that plan is essential. "The biggest obstacle to long-term return for most people is their emotional state in a downturn," McNeill said.

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To create a plan, focus on goals -- not return. That is, what do you want to do with your money? If the goal is to pay your kid's college costs, decide how much money you need and how much risk you're willing to take to get there. If you don't mind your student borrowing money in a worst-case scenario, then maybe it's OK to take on risk with your invested dollars. If student debt is out of the question, don't take that risk.

If you don't have a financial planner, consider getting help online to develop your plan. Vanguard, Charles Schwab, Fidelity Investments and others offer research and tools on their sites. MarketRiders is an online portfolio manager. ESPlanner is a do-it-yourself planning tool that takes a holistic look at your financial picture.

The stock market is "like a roller coaster," Bergman said. "It's going to move around quite a bit, but in the end it's the best way to grow capital. There are two ways to approach that. One is to watch it minute by minute, day by day and make yourself nauseous."

The other is "to create a plan with diversified investments, making sure you have your goals identified, making sure it fits within your risk tolerance, making sure your cash-flow needs are met," Bergman said. "Then let it go."