Graduate training in economics requires more mathematical sophistication than undergraduate economics. Students interested in pursuing graduate studies in economics will need to study more advanced mathematics. Math courses that are particularly appropriate for Ph.D. study in economics include Math 101 and 102, Calculus with Analytical Geometry; Math 201, Multivariable Calculus, Math 203, Linear Algebra; Math 205, Theory of Probability and Applications; Math 210, Differential Equations with Applications; and Math 301 and 302, Introduction to Real Analysis. You may want to consider a minor or double major in math.
The other “preparation” you should keep in mind concerns letters of recommendation. You will need strong recommendations to get into a competitive Ph.D. program. That will require that your undergraduate professors know you well. It’s up to you to make sure that they do. Taking multiple courses with a professor helps. Study away works against this.
Should you go straight into a Ph.D. program? Not necessarily. The opportunity cost of a Ph.D. program is high. We recommend that most students spend a couple of years working with economists, learning what Ph.D. economists do and seeing if economics research is something that drives you. That experience can help you determine whether getting a Ph.D. in economics is worth its high opportunity cost.
Where should you apply? Admission to top Ph.D. programs in economics is extremely competitive. You should not underestimate how tough it is to get in and should plan to apply to several back-up programs. There are several rankings available on the web that can give you a rough idea of the reputations of various departments. Most departments are stronger in some fields than others, so you should talk with a professor in the field you are interested in to get specific suggestions on where to apply.
Other valuable resources:
Check out the American Economics Association's useful website for grad school resources: http://www.vanderbilt.edu/AEA/gradstudents
Harvard University professor Susan Athey has extremely useful tips here: http://kuznets.harvard.edu/~athey/gradadv.html
Mark Foley at Davidson College hosts a page with a wealth of advice and information here: http://www.davidson.edu/academic/economics/foley/gradschool.htm