Acceptance to a PHD program can be extremely competitive. Most top programs only accept less than 10 percent of applicants. Therefore, it is a good idea to apply to at least 15 PHD programs to better your chances of gaining acceptance. Here are some general rules for applying to a PHD program. Note that every field of study is different and might have unique requirements (e.g. subject GRE scores, course prerequisites).
First, ask yourself why you want to go into a PHD program. Do you know what work is required as a doctoral student? Doctoral programs can be extremely rigorous. Are you prepared to read thousands of pages each week, write a dissertation, and become a research level scientist or scholar? Also, a PHD is generally considered to be a "de facto" union card for an academic position. If you are not interested in teaching college or conducting research after obtaining your degree, a PHD might not be for you. Not all PHDs teach or conduct research, but the majority of doctorate recipients work within academia.
Choose a graduate program according to your desired supervisor's reputation and not simply the reputation of the program. Your supervisor will determine your success in the program. No matter how qualified you are to study at a prestigious program, if your specific research interests do not "fit" with a specific faculty member, admissions is unlikely. Try to contact specific supervisors to establish a relationship with them.
Obtain research or job-related experience. This will prove to graduate programs that you are capable of doing research without excessive training. Also, you might get a feel for graduate-level research. Examples might be assisting a professor in a clinical study, writing an honor's thesis, or working as a teaching assistant. Also, presenting at regional, national, or international conferences (even as a student poster) can show your abilities and academic interests.
Take the GRE. Make sure that you take it in time to meet application deadlines. Some graduate programs require GRE subject-tests in addition to the general test. Most programs have certain cut-offs for GRE scores, so make sure to obtain a score required by your program of choice. Note that an economics program will prefer a high math score and a history program will prefer a high verbal score. Almost all programs prefer a high GPA to high GRE scores, but a low GRE score can keep you out of a competitive program.
Do things required by certain programs. If you want to enter a PHD program in economics, you might have to take undergraduate courses in calculus. Keep your grades as high as possible. If a program requires a masters degree, obtain a masters first or ensure that you will be done with your masters before enrolling in a PHD program.