For an interview I would say nice slacks or a skirt, with a conservative blouse. A finishing piece like a sweater or a simple jacket is also an option. It is also important to remember to go easy on the perfume and cosmetics. Whether you wear flats or heeled shoes, make sure you can walk in them!
I work on a college campus helping students dress for interviews, and I always remind my clients that it is important to present yourself confidently, and let the focus be on your credentials. You want them to remember you, and not your outfit! So don't go too extreme in either direction -- khakis and a polo are a bit too casual, and a suit is too dressy!