What are some guidelines to keep in mind when selecting members of one's PhD committee?
There are some general guidelines for choosing members of a PhD committee:


They should have some enthusiasm for the work that you do. If they're not invested in seeing you succeed, you won't get as much utility out of that committee member as someone else who might not be as knowledgeable, but is more committed.
They should have the time to be on your committee. If they're extremely busy, then it doesn't help to have them on a committee, because it will be difficult for them to attend the meetings—or it will make scheduling the meetings a nightmare.
They should not have conflicts with either you or your advisor. A thesis committee is already a somewhat political body. There's no need to add extra politics to the situation by having interpersonal or professional conflicts before the committee even begins to meet!
Collectively, there should not be a power "imbalance." If your advisor is a new assistant professor, don't overload the committee with a bunch of full professors holding named chairs, and vice versa.


When they will set preliminary exams, there's the additional qualification of:


The committee members should be familiar enough with your area that they know what it might make sense to test you on, but not so familiar with your work that they turn it into a "gotcha" game.