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.