The largest gains often occur when they are least expected.

"Consider the case of an investor who found out that he was overconfident of his ability to stand the stress of the kind of bear market we had in 2008. By November 20 of that year, he realized he had overestimated his willingness to take risk. He sold out of stocks with the S&P 500 closing at 752. His plan was to wait until the market had been up for more than 30 days (or when the green flag would be up).

With the S&P 500 closing at 903 at the end of the year (and having missed out on a rally of 20 percent), he buys again, believing that it was now safe to get back in. Unfortunately, the market dropped another 25 percent by March 9 and he had enough. Do you think this investor will ever be able to buy again? And of course, unless he had, he missed the greatest market rally in 70 years. One of the problems with market timing is you have to be right twice, not just once."