My father's most frequent piece of wisdom comes from a Harvard Business School study that he quotes regularly. Its not intelligence or money or ego. What the study discovered is that two things contribute most significantly:
A) Perseverance: A person who sticks to it and works on things through thick and thin will accomplish them.
B) A steady personality: People who doesn't become overly elated when they succeed or overly depressed when they fail. They take the good and the bad and continue to move forward no matter what obstacles or triumph.