I recently spoke at a job-training seminar about appropriate clothing to wear in the workplace. To summarize for those of you not lucky enough to attend :> (Tips apply for people working in offices or going on interviews anywhere.) ....
Men are easy - good quality, well-fitting dark suits (black, navy, or grey, or tiny pin-striped versions of those colors), crisp, clean, pressed shirts (white or colors) with all buttons present, matching, and fastened, and a silk tie in complementary colors. If you aren't sure what well-fitting means, ask a tailor or... a sales clerk in a posh men's store (even if that isn't where you bought your suit).
Silk ties are a must. Polyester or rayon is NOT a substitute!
Remember, it is worth investing in a good-quality dark suit an tie. Sure it's more expensive than the cheap stuff, but it looks better, lasts longer, and can be worn on interviews, at work, to weddings or funerals, on dates, and really for just about anything. So save up if you have to, but invest in that good suit. With some careful selection, you can pick a suit that will be relatively impervious to fashion trends and will look good year after year.
Next up, women's wear!