You already have their attention and they already value your skills and experience. The interview is your chance to continue to impress them. You need to create a positive and professional impression to give yourself a competitive edge.
Generally for graduate positions, a business suit is most appropriate. However, organisations have different standards of dress. Research corporate images, photographs and successful employees as a guide to dressing appropriately. Choose styles and colours that do not distract others and that are not distracting for you. They should fit well, be in good repair, and be neatly pressed.
Keep all electrical accessories (phone, organiser, pager etc) turned off and out of sight. Turn off watch alarms. Shoes, belts and bags should be clean and in good repair. Shoes should be practical, comfortable and polished (avoid stilettos and sandals with socks). Check that your heels are not worn. Less is best for jewellery, bags, scarves and hair accessories. Ties, generally, should be professional and conservative. Avoid garish, humorous and stained ties.
Clean, neat and simple. Pay attention to possible details such as loose hair, chipped nail polish and dirty fingernails. Make sure that any aftershave/perfume you wear is not overpowering as some people are quite sensitive to these products.