Buy and hodl if you don't know how to trade.
Never sell your coins with a loss.
it didn't work all time
some people bought bitcoin when it was at peak(around 1200$)
so the only option they had either sell at loss of hold for year with hope of price increasing
Well you need certain notions of spoting bubbles. the 1200 peak was a big obvious bubble. Would anyone guessed that it would crash this much? not really, but it was obvious that at least a big correction would happen.