One of the most important decisions every doctoral student makes is who to pick as a doctoral advisor. If possible, doctoral students should think about possibilities for a doctoral advisor and members of the doctoral/dissertation committee before selecting and starting a program. Who you choose as your advisor affects so many parts of your life during your doctoral studies, such as the opportunities you have for funding and whether you get feedback on your research or scholarship in a timely manner.
One tip is to think about how well the faculty member communicates and responds to communication. For example, does the faculty member respond to your emails and/or phone calls? Is the faculty member available to meet on a regular basis? How long does it take him or her to respond to your communication? These are all questions that you should think about carefully because you need to get feedback from your advisor on a regular basis during the dissertation writing stage, and you don't want to find yourself working with an advisor who doesn't seem concerned with helping you move forward in your program. You also want to work with a faculty member who has demonstrated that they are esteemed and have been productive in their field, because learning from him or her will help you become stronger in your field.
Who you choose as an advisor is crucial, and can even affect whether you are selected to interview for a faculty position. Search committees look highly upon good mentors—those who are helpful to students and have a good record in terms of productivity in research and scholarship. There are good and bad advisors in any department and university, and there can be a lot of negative effects of choosing a bad one. Thus, it is a choice that should be given a lot of thought, and a considerable amount of time should be spent communicating with members of the department to make sure the right choice is made.
Talking with other graduate students in your program as early as possible is one of the best things you can do, as other students will have had direct contact with the faculty members and may have even heard stories from students who have graduated. However, keep in mind that the advisor/graduate student relationship can often be affected by personalities, so one negative story does not mean that a particular faculty member is not the right choice for you as an advisor. Choosing a doctoral advisor is possibly the most important decision you make as a doctoral student, so it is something that should be given as much thought as possible.