"Don't fight the tape" is one of the oldest adages in financial markets. It means you should accept the market's trend, regardless of whether you think it's justified.
Former Forbes Senior Editor Robert Lenzner believes the idea certainly applies now, as stocks approach their seventh year of a bull market.
"Don’t fight the market, because up markets in stocks last far longer in length than down markets," he writes in an article for Forbes. "Over time the bulls have it all over the bears according to the facts of history."
Bull markets on average last about 97 months each, according to Morningstar. This bull market, which began in March 2009, has lasted almost 70 months.
"By comparison, bear markets since the 1930s have an average duration of only 18 months," Lenzner says.
"This suggests that not including the period of the Great Depression, when the market index lost 83 percent of its value, that if you had the staying power and could not possibly pick the market bottom, you were far better off sticking it out through the bad times."
Meanwhile, Jack Ablin, chief investment officer at BMO Private Bank, thinks the bull market has further room to run, just like the market in the late 1990s, but that we could be approaching a bubble similar to 1999-2000.
Just like the 1990s, commodity prices are falling, the dollar is rising, the U.S. economy is stronger than many economies overseas, and there's massive foreign inflow to stocks, he tells
CNBC.
All of this sets the stage for higher stock prices this year, Ablin said. The S&P 500 climbed 11.5 percent last year.
"If the price-to-sales ratio — which is around 1.7, 1.8 now — gets to be over 2 times, then I would say we're in that same bubble territory we were at in the tech bubble of 1999, 2000," he notes.