This week, the Wall Street Journal had an interesting piece on the pitfalls of business casual attire. And it’s a big pit, this Casual Friday dress code; one image consultant referred to it as “the black hole of style.” Employees are often confused about what precisely is appropriate for the Casual Friday office, and they wind up wearing the wrong thing more often than not.

The WSJ points out that the rules of business casual often vary dramatically from place to place:

Consider the progress of Neonu Jewell, export compliance counsel with Accenture in Chicago. At a Fortune 500 hospitality company where she used to work, a dress code barred denim — so some people wore sweats. At her next job at a Washington law firm, senior attorneys were disapproving of women who wore open-toed shoes on summer Fridays. At Accenture, 37-year-old Ms. Jewell says, business casual still involves suits or at least carefully maintained slacks, shirts and blouses, which she says reflects the highly professional work environment there.

That bit about people wearing sweats to the office of a Fortune 500 company made me die a little inside. Seriously.

While the specific rules about what is appropriate from one workplace to the next may vary, there are some general rules that you can — and should — apply to any business situation. Because like it or not, your appearance influences people’s perceptions of you.

Outfits available at Ann Taylor and Talbots.

1. No visible underwear, ever. “People think ‘it’s OK to wear a tank top with bra straps showing because the tank’s Dolce & Gabbana,’ says Alicia Kan, global head of communications for Synovate, a market-research unit of London’s Aegis Group PLC.” Your bra — straps and all — should be completely covered up for the office, as should your thong or briefs or whatever you have going on under your skirt. Always.

2. No wrinkles. The iron is your friend, as are wrinkle releasers and non-crease fabrics. If you just can’t bear to iron, make some other sort of effort to keep clothes neat: pull pieces from the dryer and hang while they are still warm, or dry only until damp and then hang. Or go with knits or polyester or silk jersey, which never wrinkles. A rumpled blouse says that you just rolled out of bed an aren’t really ready or willing to be here. Not the message you want to send, even on casual Friday.

3. No shorts. Shorts are for casual wear only, not for the office. And yes, even those dressy “city shorts” are too casual, because they are SHORTS. When it’s hot, opt for a skirt instead; a simple cotton skirt (ironed, natch) is still more professional than the most tailored dress shorts.

4. Nothing strappy or strapless or mini. A sleeveless shell is fine, as long as your bra stays covered; a skirt that hits just above the knees is fine, as long as you’re not flashing the room when you sit down. A sheath dress is perfect, as long as you can get a bra under it (and keep it there). But any pieces than can go directly from the office to a poolside barbecue are too casual for the office and should be saved for the barbecue.

What SHOULD you wear? Choose casual pieces with clean, tailored lines and quality craftsmanship. Mid-range chains like the Gap are good places to look for casual office basics (chinos, skirts, button-up shirts). And don’t skip accessories just because you are casual; a great necklace or bright pair of shoes can dress up the most basic khakis-and-twinset outfit. Remember that you are going to WORK; dress like a professional and people will treat you like one, even on the most relaxed of Fridays.