In the school of fashion, as we've learned from both magazines and mothers, there are in's and there are out's. Little black dresses: good. A black bra as your evening look: bad. Classic white shirt: good. A head-to-toe white outfit: bad (unless you're going to a kabbalah party). Louis Vuitton bag: good. Fake Louis Vuitton bag: bad.

We know these things deep down, but sometimes irrationality takes hold. Something grabs your fancy: the baggy plaid dress that reminds you of that amaaazing night at the Nirvana concert in 1990, the all-leopard look that is so Studio 54, the skirt that might be a little short — but short's cool, isn't it? How can it be wrong, you think, when it feels so right?

But it's not just wrong, it's a crime. So take these tips to heart unless you want to end up in an on-the-stand stare-down with the D.A. And unlike the guilty gal on this page, you won't have a chic orange Ralph Lauren ensemble waiting for you in the slammer.

ALL-LEATHER LOOKS

Yes, leather is in. For example, Yves Saint Laurent was having such a leather moment that designer Stefano Pilati turned it into both a bodysuit and a jumpsuit. Michael Kors's strapless leather dress was a runway sensation, and even more so when leather lover extraordinaire Angelina Jolie wore it on the red carpet. But leather is best used wisely. If you wear a skirt, tone it down on the top. A jacket? Jeans or trousers. If you're feeling a bit Gwyneth or Chloë Sevigny, a saucy pair of shorts might just do it. But never wear the whole lot together, because the dominatrix should stay in the bedroom (if you're inclined that way, of course), not the boardroom.

CHEEKY HEADPIECES

Legendary fashion editor Anna Piaggi is the only, repeat only, woman in the world who can get away with a headpiece — and note, hers are eccentric, not cheeky. Headpieces are worn to great effect on the runway to provide theatricality. Operative word: theatrical. So, unless you are about to hit center stage at the Metropolitan Opera or are the lead in Shrek, headpieces are not for you. A girl can be cheeky in a headband, but not with headgear.

THE POLITE PUMP

It's not a season for mumsy shoes. It's a season for shoes that you may need Olympic training to wear (exhibit A: the Louis Vuitton boots our murderess literally killed for). We are talking eight inches, platforms — assault weapons if moved from foot to hand. These are tough times, not demure times. Buy footwear accordingly.

HIPPIE DRESSING

Just because Woodstock had its 40th anniversary recently doesn't mean you need to look like you were there. (So aging!) And Sienna Miller circa 2005 is not worthy of a retrospective just yet. Keep those threads and beads and fringes in your closet for the next costume party. And braids? Don't even ...

RUNWAY LOOKS HEAD TO TOE

One of the season's most photographed looks was the fetching red velvet brocade coat Miuccia Prada designed. It was accessorized on the runway with a large pair of waders. During the show, they looked wondrous. On 57th Street during rush hour, maybe not. Find a seasonal piece that speaks to you and make it work. That coat would look just great with jeans, wouldn't it?

SLOGAN TEES

"Yes we can" was pretty catchy, and it meant something too. Everything Marc Jacobs throws on a T-shirt is devilishly effective (especially when celebs take their kit off for charity), but slogan tees should more often than not be thrown into the penalty box. You're clever and sardonic? Prove it with your personality, not your shirt.

IRONIC GRANDMA DRESSING

We love our grandparents, we really do. They give us cake, tell us stories, and pat our heads when we are sad. They don't commonly, however, serve as our fashion muses (unless, of course, Jane Birkin is your grandma). And that's okay; leave grandmas alone. Also, and this is vitally important, if you are a single girl about town, you will never get a date dressing like this. Housedresses, chunky shoes, and glasses do not say it's sexy time.

OVERSIZE BAGS

If the Olsens are over this look, you should be too. Nobody has that much to carry unless she's going away for the weekend or is the littlest hobo. A bag as big as your body? That's a body bag.

THE '80S IF YOU'RE 80

The first time around you were 50, and it didn't make sense then, either. Leave the resurgence of '80s looks — the shoulders, the minis, the Dynasty sequins — to the twenty-somethings who were born then. They think shoulder pads are new and exciting. The only workable alibi? If you were one of the girls in Robert Palmer's "Addicted to Love" video and you've shopped your closet.