An investor will succeed by coupling good business judgment with an ability to insulate his thoughts and behavior from the super-contagious emotions that swirl about the marketplace.
The worst sort of business is one that grows rapidly, requires significant capital to engender the growth, and then earns little or no money. Think airlines. Here a durable competitive advantage has proven elusive ever since the days of the Wright Brothers. Indeed, if a farsighted capitalist had been present at Kitty Hawk, he would have done his successors a huge favor by shooting Orville down.
There is no question that an important service is provided to investors by investment companies, investment advisors, trust departments, etc. This service revolves around the attainment of adequate diversification, the preservation of a long-term outlook, the ease of handling investment decisions and mechanics, and most importantly, the avoidance of the patently inferior investment techniques which seem to entice some individuals.
I always say that in investing you want to buy stock in a company that has a business that's so good that an idiot can run it, because sooner or later one will. We have a country like that.