Greg Mankiw has some Advice for Aspiring Economists:
Here is some advice for, say, an undergraduate considering a career as an economist.
1. Take as many math and statistics courses as you can stomach.
2. Choose your economics courses from professors who are passionate about the field and care about teaching. Ignore the particular topics covered when choosing courses. All parts of economics can be made interesting, or deadly dull, depending on the instructor.
3. Use your summers to experience economics from different perspectives. Spend one working as a research assistant for a professor, one working in a policy job in government, and one working in the private sector.
4. Read economics for fun in your spare time. To get you started, here is a list of recommended readings.
5. Follow economics news. The best weekly is The Economist. The best daily is the Wall Street Journal.
6. If you are at a research university, attend the economic research seminars at your school about once a week. You may not understand the discussions at first, because they may seem too technical, but you will pick up more than you know, and eventually you’ll be giving the seminar yourself.
Mankiw also cites his American Economist article, My Rules of Thumb (PDF). It contains this telling comments:
Throughout my life, I have been blessed with broad interests. ...My broad interests (short attention span) help to explain my diverse (incoherent) body of work. My research spans across much of economics. Within macroeconomics, I have published papers on price adjustment, consumer behavior, asset pricing, fiscal policy, monetary policy, and economic growth. I have even ventured outside of macroeconomics and published papers on fertility with imperfect birth control, the taxation of fringe benefits, entry into imperfectly competitive markets, and the demographic determinants of housing demand. None of this is part of a grand plan. At any moment, I work on whatever then interests me most.
Of course, breadth has its costs. One is that it makes writing grant proposals more difficult. I am always tempted to write, "I want to spend the next few years doing whatever I feel like doing. Please send me money so I can do so." Yet, in most cases, those giving out grant money want at least the pretense of a long-term research plan.
The greatest cost of breadth, however, is lack of depth. I sometimes fear that because I work in so many different areas, each line of work is more superficial than it otherwise would be. Careful choice of co-authors can solve this problem to some extent, but not completely. I am always certain that whatever topic I am working on at that moment, someone else has spent many more hours thinking about it than I have.
There is something to be said for devoting a lifetime to mastering a single subject. But it won’t be my lifetime. I just don't have the temperament for it.
I know exactly what he means!