I've been to four interviews, and they've all been different. I've seen a girl in a denim dress, a few girls in slacks, and I've seen A LOT of navy and black suits. One administrator of a med school said we looked like a bunch of morticians. I wore a deep blue (not navy) suit with a long skirt, off-white blouse, pearls, and taupe suede shoes. I made sure the shoes would be comfortable before I bought them. You'll do a lot of walking. I have almost shoulder-length hair, and I wore it down each time, but neatly done.
My advice: wear what you think expresses your personal sense of professionalism. Then you'll be comfortable with how you look and come off as being confident and mature. It's okay to be a little different from the crowd, but not too much. You want to stand out for your qualities, and a small amount of individualism in your dress can help you accomplish that.
Hope that helps.
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We are the music makers, and we are the dreamers of the dreams.
--Willy Wonka