This is one of those classic aphorisms in trading. I’m heeding it at the moment, only doing short trades given that ETH/BTC has been trending down for nearly two years. So, if the price goes up a bit (technically a retracement in a downward trend) I sell some borrowed ETH for BTC, and when it drops a couple of percent I buy the ETH back again. Backtesting with my XGBoost model over the past year gives approximately 100% profit doing this, on recently updated prices.
Since most of my schemes make some money for a while and then lose a lot I’m not putting too much capital into this. I make $100 here and there. If it remains profitable for six to twelve months I might consider increasing my positions. Probably by then the strategy will stop working. My pessimism comes from experience. Any sensible person would have given up by now.