It is easy to say but hard to do. I think the strategy of buying low, selling high is the most common effective strategy that most investors tend to forget. Some have their emotions affecting their strategy when bloodbath and better days come. Whenever the market turns to downtrend, they are at risk at panic selling. Whenever they are gaining profit, greed also affects them. For long term, cost averaging method is still the recommended strategy.
Many people try to practice it every day and yet we see that there are more losers than winners in this market, but the rate of winners and losers is tilted to the losers in an extreme way since more than 80% of the people are losers which means that only 1 in 5 are traders that make profits, this show to us how hard trading really is.