How do you choose this group of people who will determine your fate? Carefully!
Ask Your Advisor
Speak with your advisor/mentor to learn about how students in your program go about assembling a committee. If you do not yet have an advisor, that's your first step -- locating someone with whom to work. Once you have a mentor/advisor, he or she can tell you about the local norms.
Get Folks Your Advisor Likes
When it comes to which faculty to invite to sit on your committee, seek your mentor's advice because you need someone who you mentor feels he or she can work with. Also your mentor will have info about the faculty's history (e.g., if he or she is contentious). Dissertations are not just about a student completing a degree - they're about politics too. Select a faculty member who doesn't get along with your advisor and you may have a committee member who is difficult and just plain hard to work with -- and who finds fault with your work simply to get under your advisor's skin. It happens - and this kind of situation can slow down your dissertation and keep you in grad school longer than you want or deserve to be.
Learn From Other Students
Seek input from other students as to how they secured a committee, what kinds of things they looked for, and their experience with particular faculty. Some faculty are flaky. They miss meetings, forget to read your work, and run behind. They may be very nice, helpful, and easy to get along with, but they can interfere with your progress.
Trust Your Instincts
Get to know faculty throughout your grad school years. As you take classes, talk with faculty, and watch faculty interact, keep the dissertation in mind. Trust your gut. If someone seems like a terrible choice, even if he or she is a very successful professional, you should trust your gut and think twice before asking him or her to sit on your committee. This doesn't mean that you should exclude successful, but difficult, people. It simply means that you should go in with your eyes wide open.
Knowing who to choose for your committee really comes down to communication -- getting to know faculty who you will invite to be on your committee, communicating with your mentor about your needs, his or her needs, and potential candidates, and communicating with other students about their observations, experiences, and history.