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.
Agreed, but i think it also happened in bitcoin,ethereum or to all coins in general specially on bear market when all coins are on downtrend position. When you bought at the dip thinking that is the buttom but the price still go lower, so it is better to do when the price of a coin you want to buy is in downtrend dont use your all funds to buy all at once better to buy it slowly and small amount like 25% of your funds per purchase.