The simple rule is "buy at the bottom and sell at the top". But i don't think this applies to all coins, it only applies to solid coins like bitcoin, ethereum and the likes, because they have stood the test of time and would surely find their way back up.
I disagree with this because you could definitely always buy at the bottom and sell it at the top but the challenge is
when you will buy it. This should already be a common rule for everyone. If you check all the charts of the coins, there would always be a lower low and a higher high. What you would do as a trader is when you are going to trade, what indicators tell you to trade, limit positions, limit risks, and many more. It's not going to always be about FOMO, it's about being smart with what you are trading.