Most people will make you believe that if you are good in your subject, you have a great chance in succeeding the PhD grind. And I agree with that, only that I do not believe that it is all you need.
There are a few things that a candidate needs to have to be able to go thru the grind and come out a success -
- Basics of the field of study should be thorough and clear
- Definitely need to have strong quantitative skills... if you think you don't have them, start building them right now
- Professional experience may come into play depending on the field of your study, it is never harmful though
Some of the common subjects one should know are Mathematics and Economics for their generic use throughout the course.
So, get the basics right and build your quantitative skills before you are ready to jump into the pool.
Best of luck!
Mayank Srivastava
General Education For GRE for MS/PhD Application Assistance