Over the years, I've learned it's more effective to: buy when there is doom and gloom (blood on the street), sell when there is nothing but good news (about the company, industry).
For example, when there is a crash and everyone is saying: wait some more, the bottom is not reached yet, it's all going to shit, then buy.
You should also always buy in steps, buy 30% now, wait some more, buy another 30%, wait some more, buy remaining 40%. This helps tremendously with your nerves and emotions.