Great pair of jeans, fitted tshirt and good heels - ones you can walk in but take off easily for if they measure you.
Natural hair and makeup - dont believe the "go in with no makeup" thing as it's lies, go in with say a tinted moisturiser/light coverage foundation, highlighter, subtle bronzer, natural blended shadows and mascara.
Done dozen of castings with top agencies and clients,just look cool and classy, be interested in what they do, clients love that! Show pics that relate to the job,all different and not too many.Be yourself and enthusiasm is a winner!!