This might be the best advice for stock market investors going into 2016

It can unsettling to see the stock market go through big bouts of volatility like it did in 2015.

With the stock market basically flat for the year, it's easy to think that now's an opportunity to cash out of the market and get back in after the next sell-off occurs.

Unfortunately, markets are not that predictable. And while it doesn't feel good to lose money when the market falls, it isn't exactly great to be missing out when the market gains.

Gluskin Sheff chief economist David Rosenberg circulated a short note on Thursday briefly summarizing 2015 and looking forward to 2016. At the end of the note, Rosenberg quoted a Wall Street Journal op-ed from Tuesday that reinforces one of his favorite investing themes: Stay steady, stay diversified, and don't try to time the market.

The WSJ op-ed Rosenberg quoted concludes with this (emphasis ours):

"U.S. securities markets are highly priced at the start of 2016, and future returns will most likely be lower than in the past. But the timeless lessons-keep invested, don't try to time the market, and diversify broadly-remain the best guide for investors."

Even though markets may look pretty expensive, it's still a better idea to think long-term, and to keep a steady hand in your investments.