Carnegie kept production costs lower than selling costs making income much higher than what he was paying for.
He was always aware of his money and how well his business was doing. This is very wise because businesses were very intense and if something unexpected happened, he would be prepared.
Carnegie made sure that he donated his money to education, charities, etc. He believed that his money should be put to use, not just for himself.
He was a master of vertical integration. Vertical Integration is the control of processing raw materials to finished products that can be manufactured and sold.
"He made many wise choices and found that his investments, especially those in oil, brought in substantial returns" ("Andrew Carnegie Biography")
Carnegie was important figure in his time and has affected our lives, too.