Investors should buy and hold stocks, which will generate an average annual return — including dividends — of around 7 percent over the long term, says John C. Bogle, founder of the Vanguard Group.

"Your money will double in 10 years," Bogle tells the Wall Street Journal.

"How bad is that? People ought to get over the illusion [of higher expectations] and realize that they may have to invest for longer time periods, start earlier and save more."

Buy and hold refers to all assets, including bonds.

John Bogle (Vanguard file photo)

"Diversification is not only the first important thing investors should think about, but the second and the third, and probably the fourth and fifth, too," Bogle says.

Stocks have taken a wild ride for over a month now.

The debt-ceiling impasse and subsequent Standard and Poor's downgrade of U.S. ratings in August coupled with European debt woes have sent stock prices pummeling, while the sporadic items of good news have often been enough to bring in bottom fishers and boost equities indices into the clouds.

Expect volatility to stick around, experts say, especially on uncertainty in Europe, where concerns abound that debt problems in peripheral countries like Greece will spread across the continent and inflict serious damage to the financial system there and possibly worldwide.

"Europe is the dominant story that continues to be behind the volatility; there is not a clear path to recovery there that has led to making the market nervous," says Brad Sorensen, director of market and sector analysis at Charles Schwab, according to MarketWatch.