Research schools to determine which ones have the top programs in the field you intend to enter. One of the best sources for this is the regular rankings reports compiled by "U.S. News & World Report." The publication provides a fairly reliable guide as to which programs have the best reputation among the academic community.
Determine what your area of specialization will be within your intended field of study. This should be based on the area in which you plan on conducting the majority of your research. You will be pursuing this research as a lifelong endeavor, so you need to be clear about which direction you are heading.
Research scholars who also conduct research in your intended area of specialization. When you choose a Ph.D. program, choose it largely based on which scholars work for that university because you will work closely with a faculty mentor throughout the duration of your studies. Your research should closely align with that person's specialization so you can glean advice from him regarding your research and even assist him in his. Look for scholars who have a reputation as top scholars in their respective fields who have multiple publications and awards for some of their research. If you are pursuing study in a science or medical field, consider finding scholars who have written several successful grant proposals -- they will be able to mentor you to do the same.
Contact the scholars with whom you think you might like to work. Ask them questions about their research and whether they would be willing to work with you to develop your research as well. Unless they have a waiting list of students with whom they will be working, most scholars will be willing to take you on as their apprentice.
Narrow the list of potential schools and weigh the pros and cons of each program. Aside from finding scholars in your field, you should also factor considerations such as cost, financial aid, if the school has offered you a teaching assistantship, the program's reputation and the placement rate of its graduates. Choose the program that meets the greatest number of your most important needs.