This is a variant of the question James Buchanan would often ask a graduate student. The "correct" answer was of course to make a lasting scientific contribution --- to be read not in your lifetime, but centuries after your death.
Among economists perhaps nobody exemplified the opposite of this in the 20th century more so than John Kenneth Galbraith, who passed away on Saturday. The NYT obituary does an excellent job conveying Galbraith's influence on our political and intellectual culture. The claim is made that Galbraith is the most widely read economist in the history of the discipline. Richard Parker's biography, John Kenneth Galbraith: His Life, His Politics, His Economics is also an excellent source for studying the impact of Galbraith had on the intellectual and policy debate.
But Galbraith's claim to scientific influence is dubious at best. Parker tries to make the case that Galbraith's ideas are now influencing modern economic discussions in the scientific literature like behavioral economics. Among heterodox economists Galbraith is no doubt an icon, but as someone who made a lasting contribution to the mainline of the discipline it is hard to agree with Parker. Galbraith achieved influence and fame, but did not make a lasting contribution to the advance of the discipline of economics. He did set a standard for writing that we should aspire to achieve, but his core ideas are either restated Veblen, warmed over Keynes, or Marxist platitudes. It is not clear that any of his ideas are original enough to warrant that he be placed in that company of critics of the market economy. But there can be little doubt that between 1950-1970, he perhaps more than anyone popularized the teachings of Marx, Veblen and Keynes and made them acceptable to generations of Harvard students and members of the intelligentsia in the English speaking world.
On a personal note, I met Galbraith and shared dinner with him one night in the early 1990s. I was prepared to hate the evening only to be completely charmed by the man and his stories of JFK and India, of battles with Milton Friedman and William F. Buckley, or a profession which has succumbed to too much formalism, disrespect for history, and an inability to address the institutional contingencies of our age. It was an amazing evening and it was one of the important moments which taught me that you cannot divide the world neatly into those who are stupid, those who are evil, and those who agree with me. Instead the world is full of charming, brilliant, and good hearted individuals who just happen to hold opinions opposite of the ones I do. Understanding results from delving into those reasons for differing opinions without succumbing to the cheap tricks of attributing disagreement to bad motives and to lack of intelligence among opponents.
BTW, my very first professional paper in economics (first written in 1984, but published in 1989) was inspired by reading Galbraith and attempting to negotiate between Galbraith and the institutionalists on the one hand, and the Austrians on the other.