Never sell is the most bullshit thing I heard from crypto people because most of them that spouting that nonsense are those who just want quick profit and they are using that word for shilling purposes. We are all buying Bitcoin to invest and earn in the future so no one in the right mind who will not sell there holdings if they already have a good profit. They should just throwaway there private key if they really want to not sell. 

The best way to do is to read the trend in longer time frame. Buy when there’s an obvious bear trend in longer time frame and vice versa. Don’t trade using short time frame because you will be just confused and panic to the price swings of Bitcoin.