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. But most of these new coins, if you buy at the bottom, the price might still go lower and keep you trapped there. so it is better to wait and see a clear move up before you get on the train to avoid fake outs.
That rule is the basic rule in trading, but unfortunately, many people don't follow the rule because while they trade, they see the price start rally and touch the high price. They become greedy and don't want to close their trade, but they want more profit, which is not always happening. Instead, to sell at a high price at that moment, they are still waiting for the price to increase higher, and what they see in the next minute is the price is back to the low price, and they lose that opportunity.
It is why you need to have a target sell price and place an order sell, so when the price can touch your target price, it will automatically execute the order. Using that, you don't miss the chance to sell at a high price, and you can wait for another opportunity to buy back the coin. If you are afraid because the price can go down for more, you need to split your order buy into a few orders, so when the price down, you can still get the coins at a lower price.