Get your emotions out of investing, and only plan for the very, very “long-term,” Vanguard Group founder Jack Bogle advises.

He urged investors to not flee the market when it's down, noting that's been his “winning strategy."

And his winning formula? Bogle advises that individuals have a 60 percent to 40 percent stock-to-bond ratio in their asset allocation. "If you're younger, a lot higher, if you're older ... somewhat lower," he told CNBC.

He said it’s too difficult to overcome your emotions when investing in a volatile market.
“We just don't know how to do it. That's why we have said once and twice and three times, and a thousand times: ‘Stay the course,’” he recently told CNBC, once again repeating his investment mantra.

He explained that too many investors waste time and energy, for example, trying to guess if the market has hit a bottom in any given particular selloff. He said you shouldn’t fret over such factors.

“The only thing that worries me about the markets is they do so many things that are kind of crazy, and I always worry in the back of my mind about crowd psychology, all of a sudden thinking something is so much worse than it really is,” he said.

“Many investors are leaning here and all seem to want to go over the cliff and into the water together. And so that worries me. Hasn't happened now. It probably happened in one of my three bear markets at the bottom. And I've been through three of them as well as 30 corrections, counting those three. So I know a little bit of what it is about,” he said.

“And the basic idea is don't panic, and don't worry about calling the bottom,” he said.

“No guarantees, of course, we live in a risky world. And stocks are not cheap at all. Probably fully priced a little bit. So we just have to take that into account and hope that the growth of American business will finally bail us out,” he said.

For risk averse investors who may not want to invest mostly in stocks, Boggle suggests that they look very, very long term. "In the long term, stocks are going to do better than bonds, almost inevitably," he said.

To be sure, Bogle isn't alone in advising investors to use extreme caution moving forward.

Byron Wien, vice chairman of the advisory services unit at Blackstone Group LP, warns investors to "buckle up" for more stock-market volatility.

Wien told CNBC that while he doesn't expect another steep decline in the large cap S&P 500, he does see more downside ahead. "We have further to go, but I don't think it's going to be a major downshift from present levels," he said. "I don't think we've seen the ultimate lows," he added.

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