Putting aside over 90% of all existing coins and tokens, let's talk about established ones. Being around for years, those coins have small chances of disappearing completely, so we can try to invest in them. So, here's my advice on when to start buying. Look at a coin's charts, and if its current price is around or less than 10% of what it used to be at some point in the past, we are most likely at the bottom, so there are great chances of that you can't go wrong with buying this coin.
To tell the truth, I'm pessimistic on the possibility of existence of such a strategy. Buying-low-Selling-high only looks easy to follow, but in reality you never know where the market is going from a particular point. I can assure you that many people sold a lot of their coins, "to buy more later at lower price", when Bitcoin hit $2,800+ for the first time ever in Aug 2017. But the truth is that the price was never lower than that since then.