Personally, tee shirt and jeans. No earrings, piercings, "stylish" hair (faux hawk, etc..) and no cocaine finger nails is a safe bet. When I worked Officer Recruiting, the only thing that struck a "bad" chord were the goths, right out Billy-Bob NASCAR-trailer-park-walk-ins, and people dressed like gang bangers walking in spouting, "...man, I wanna fly jets" as they're waving their hands in big oversized down jacket, sideways caps and/or wool caps in the middle of June. Seriously, I'm NOT kidding.
On the flip side, if you walk in with a blue oxford button down, khaki pants, a belt with whales on it, and canvas boat shoes, looking like a young Ed Begley Jr. out of a Land's End catalog, we'd likely vector you towards SWO, supply or INTEL.