I try to limit each holding to a small % of the total. That way if any one thing takes a hit I can ride it down to 0 without adversely affecting the total. I have been tricked into selling perfectly good companies whose price was suddenly decreasing for no apparent reason, only to see them mostly come back. When I back tested what the stock market did I found most stock do blah and some are real winners. What a shame to have say sold Microsoft or amazon early on when a blip happened..
I invest in many stock sectors (energy, housing, oil, etc. ). Often hot money jumps from one sector to another. I notice one group of stock in one sector decreasing almost exactly what another group of stock in a different sector is increasing.
Also I invest in different asset classes. Some things do not move lock step with stock. Bonds, annuities, real estate(REITs), cash, or commodities for example.
Finally I invest in different countries to limit strong or weak dollar apparent changes in overall portfolio.
If you only have say 1/5th of holdings in stock and all your stock went down half, you could still theoretically have 90% of your total nest egg.
I don't market time much unless it is play money amounts but I will sell losers in taxable accounts at year end for a loss and then reinvest that money either after the necessary time has elapsed (31 days still?) or in a different holding.
Anyhow this has worked for me in past but no one knows what the future will be. Don't know if I am lucky or good, but either works.
To keep me humble I always remember Pompeii in ancient Rome that was wiped out by the eruption of mount Vesuvius in 79AD. Even the richest and smartest merchants and money lenders there didn't avoid that final market downturn.
(not mad just like the little flame to depict the volcano..)