Buy Low, Sell High
You often hear this truism, and yet people miss opportunity after opportunity to follow this basic and simple way to invest. People are always jealous of others who have either bought at an opportune moment, or sold at an opportune moment. Yet investors fail to follow this very simple and basic rule of investing.
Why don't people buy low and sell high - because people have a basic instinct to follow the crowd. When there is fear in the land, history is ignored and fear takes over.