What’s your selling strategy?

I’m trying to understand at what point do you make the decision to sell, be it a loss or a gain. I have in the past lost a lot of money by either selling too early or too late, which I feel has happened to most people.

I’ve read that some people put a percentage as a selling point. Like if you gain over 30% you sell it at that or if the value drops over 15% you sell. Do you guys have a strategy as far as selling?