When you do find the one; that agency you admire, look up to and want to be a part of it is important then to be certain you are giving all the right signals, when on placement. Make sure that they know that you are looking for a job and that you aren't just placement surfing: going along for the ride. For me, I think people started taking me seriously when I started wearing the right things.
Every agency is different with what they do and do not consider work attire, but I'd say smart casual is the norm. Looking at graduates now I do notice a certain scruffiness. Sure, there isn't much of a budget for new clothing, I appreciate that, but low riding jeans and tattered old trainers make you like like a student and not a graduate.
It was particularly important for me because I look much younger than I am. I tried smarter clothes like blazers and smarter jeans, proper work shoes and tidy hair. Separating yourself from other graduates is important and if you look like you are serious about getting a job, they will take you seriously too.
One of my friends suggested if I wanted a job I should wear trendier clothes. I think he is wrong: you should never try and be something you are not, and if they didn't want to hire me because of my dress sense, then I didn't want to work there, anyway. But what I am saying is not a question of fashion but of portrayal of your self and the right attitude, whilst fitting in to the adult world. You are a grown up now, looking for a grown up job - now dress like you mean it.