If a New Bull Market Is Coming, Don't Make This Expensive Mistake

If there's one thing every investor learns, eventually, it's that the market goes both up and down. The swings can be dramatic and swift as Wall Street shifts between periods of greed and fear. For long-term investors, an important skill is learning to live with the inherent near-term uncertainty to benefit from the long-term growth of the market -- which is the key to avoiding this expensive mistake.

If you look at a long-term graph of the S&P 500 Index (NYSEMKT: SPY), you'll notice that it has moved fairly reliably higher. However, it is a jagged line, not a straight line. There have been many periods during which this broad stock gauge has fallen with shocking speed, and some points in time where the S&P trended slowly lower -- and others in which it just sort of sat around for a stretch doing nothing at all.

SPY data by YCharts.