Most Helpful Guy
How a girl dresses for me is very important. Granted I am a little older, so I can't speak to how guys in your age group would react. If a girl really goes all out with her wardrobe, makeup, hair, nails, etc when we go out it tells me that she is into me, and she wants me "more than a friend".
The ones that show up "too casual" for a night out raises my "Lets Just be Friends" alarm. In my mind she didn't put a lot into her appearance, and has little to no sexual intention.
Just my 2 cents. :)