6. Trading too frequently

Some investors tend to rapidly move their money in and out of stocks, but that's a short-sighted view that can hurt returns.

A University of California at Davis study of 64,615 portfolios revealed that transaction costs significantly cut into overall returns. On average, the portfolios lagged the market after trading expenses were subtracted.

Investors should take a long-term view and buy securities that they expected to hold for awhile.