Mike Moffatt at About Economics has put together an excellent series of pieces on graduate school. While he's focusing on economics Ph.D programs, I'd say that about 90% of what he says is also relevent for students considering a Ph.D in Finance. Here they are:
In Choosing A Graduate School in Economics, Moffat provides general advice, but also gives links to several other resources, like this collection of advice on applying to grad school from Stanford and this one from Georgetown University.
In Economics Grad School Atmosphere, he points out the importance of the relationships between students and faculty in grad school (and of the relationships between students). After all, it's not unusual to spend 70, 80, or 90 hours a week around the department (and the average time to completion in grad school for econ and finance is probably 5 years), so being around people you don't like is plain foolish.I love it when someone does my work for me.
Finally, in Books To Study Before Going To Graduate School In Economics, he suggests some excellent books to read prior to starting grad school. Since most good finance programs give their students a solid grounding in microeconomics, mathematics, and econometrics, they're also pretty good recommendations for soon-to-be finance Ph.D. students.