•Thou shalt never let your outfit outshine your work. (Yes, creatives and fashionistas, this applies to you as well!)
•Thou shalt abstain from clothing with obnoxious logos. (Besides Bebe logo tops being so 1999, no one wants a walking billboard in their office.)
•You shall avoid all sexy and revealing clothing. Never show more than 1 inch of cleavage or wear a skirt that skims your bum. For most offices, skirts that hit more than 3 or 4 inches above the knee are considered inappropriate (If you want to wear shorter skirts, or even dress shorts, pair them with opaque tights to temper the higher hemline). Also, no tank tops unless you have something layered over it to cover your shoulders.
•Thou shalt not wear any ripped, torn, or sloppy clothing. This includes anything that can be worn to the gym or the beach (gym shoes and flip-flops etc.).
•Thou shalt dress like my female boss. If you do not have a female boss, emulate the highest-ranking female in your company. This doesn’t mean steal her style. Rather, base your office attire on what she considers appropriate. Is she casual? Always dresses to the nines? Take a cue from the people in positions you aspire to have someday. They don’t say, “dress the part” for no reason.
•You shall always dress to impress—even on “casual Fridays.” Don’t let casual Fridays fool you. Jeans may be okay, but that old tee reserved for lazy Sundays and those hot pink flip-flops are just not going to cut it… Casual Friday attire should be based on your weekly work wear, but taken down a few notches.
•Thou shalt not don underwear as outerwear. Ever. This is pretty self-explanatory…
•You shall always use your best judgment. Consider your work environment and dress accordingly. Anything you can wear to a club should never step foot in an office. If you’re unsure about an outfit, it’s best to err on the side of caution and go with something else.
(full article here)
Here's a few example of Casual Friday done right:
I've read many times to dress for the job you want, not the one you have - and I do love that idea as well. For my job, I have to be aware of how physical my job will be that day as well. If I'm packing 40 boxes to mail to our Rams event in St. Louis - it's not a skirt or dress day. If I'm going to have to sweat in out washing water coolers as part of my post event routine, I'm not going to wear 5 inch heels. So I'm a firm believer in wearing what works for you - but that doesn't mean a compromise on dressing to impress.