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 might agree with you with this. Those trusted coins will surely go back up since it is a cycle. It will go down then it will go up then down then up.
Buying at the bottom and selling at the top is easy to say but in reality, even the professional traders don't even know which is the bottom of it. The best thing to do is to have a strategy and plans that will help you when to buy and when to sell and not buying the bottom and selling at top because it is very hard to predict.