Everyone agreed blue jeans aren't appropriate, and the norm seemed to be, for both men and women, something that comes with a jacket. Due to the variation in fields, be sure to check with people in your own department. And keep in mind, you need to find clothes that fit and are comfortable. Needless to say, you'll pack these in your carry-on luggage, rather than checking them. This is also a fine time to try out any new attire.
Some academic departments provide interview preparation for their doctoral students.
See if your department does. If not, perhaps your adviser or another faculty member will work with you. At your campus career center, you may find counselors experienced in working with graduate students who will do a mock interview with you. If such help is not available to you, figure out which questions you need to practice the most and work with a friend or family member. But you can also find helpful tips on the Web sites of scholarly associations.