I work as a manager for Pac Sun, so it's pretty much expected of me to look even younger than I am because of the clothing styles we sell. When I worked for Hollister, I dressed like the mannequins because I wanted to project the brand's image.....at Pac Sun, not so much! I *hate* being mistaken for anything less than a amanger. So, in order to still project the brand image and look a bit closer to my age (I don't look 25...I look 20!) while making it obvious that I'm not a youngin', I typically wear dark jeans, sandals, and a solid-colored tee with some minimal accessories. Honestly, it's all about what YOU are comfortable in! Personally, I feel out of place and dressed younger when I'm wearing Hollister or Abercrombie stuff, but I've found that they're "adult" brand, Ruehl, is actually quite nice. It's pretty much the same stuff, but more "grown-up" and flattering for those of us who aren't quite in the demographic for the younger stores!
I actually get all my solid-colored tees from Pac Sun. They're fantastic - lightweight, easy to layer, and they come in TONS of colors (everything from neon pink to the standard black). The price is quite nice too, and there's frequently some sort of promotion, i.e. two tee's for $20 or something like that. Even when I'm not at work, that's usually how I dress, although I might add a scarf or more interesting accessories.