A friend of mine just emailed this to me. It's from the book _Getting What You Came For_ by Robert Peters, Ph.D., and it Seems to be pretty spot-on:

Lester H. Bodian (1987), a psychologist, did his PhD on factors that influence success at getting a PhD. Here's what he found was correlated with success (with explanations by the author of the book, Dr. Robert Peters, on further examination of Bodian's psychological studies):

Being married - Married students are less likely to drop out, probably because they receive more support and encouragement.

Being on schedule - People who take longer to finish their master's programs tend not to finish their Ph.D.s

Being in the hard/natural sciences - Physical scientists and biologists are most likely to finish, social scientists lie somewhere in the middle, and students in the humanities are most likely to drop out. This pattern reflects higher levels of both collaborative research and funding in the sciences.

Being financially secure - Financial support is particularly important in determining attrition, and students who work full-time, for example, have high attrition rates.

Having clarity of purpose - Studies show that students have a better chance of finishing if they have concrete career goals and their future jobs depend on attaining the degree. Thus, doctoral students headed for university teaching are more apt to get their Ph.D.s than are students who want to teach on lower levels where a master's degree would suffice.

Having a good relationship with your advisor.

Need I say more?