Nothing looks more dated than having an all-one-length, "high school girl" ’do — unless you’re Demi Moore. Once you're past the age of 21, you’ll look prettier with a soft style that has lots of shape and movement. Think layers.

"Carefully placed banglike layers that frame your face around the eyes and cheeks will soften your features, as well as fine lines and wrinkles," says Rob Cook, salon director of a Van Michael Salon in Atlanta. If you don't love layers around your face, ask your stylist to cut shorter layers in the back section of your head but keep the front long, suggests Eva Scrivo, stylist and owner of Eva Scrivo Salon in New York City. This creates height and fullness on the top of your head and directs the eye up, which prevents long hair from making your face drag.

New Rule 2

Know How Low to Go

The days of waist-length hair are pretty much over, "but hair that goes to the shoulder or collarbone is easy to work with, simple to keep up and looks good on most people," says Scrivo. If you're petite, medium length (no longer than your shoulder) is the way to go because it can create the illusion that you’re taller. If you go any longer, though, your hair will overpower you. If you have a full figure, long hair is especially flattering since it creates a sleek line, making you look thinner.

New Rule 3

Pamper It

Long hair is more apt to look fried than shorter styles. So once you grow out your mane, be sure to get regular trims (every two to three months). Also, get a deep conditioner and use it once a week. Madora George Michael of Madison Avenue 60-Second Conditioner, $16, 212-752-1177, is a cult favorite of the long-hair set.

To reduce split ends, try a strengthening shampoo like Garnier Fructis Long & Strong Fortifying Shampoo, $4, drugstores. If you have long hair with lots of short layers, using a light pomade on the ends will separate your hair and play up its shape. Try Herbal Essences Texturizing Pomade, $3, drugstores.

New Rule 4

Know Your Texture

If you have really thin hair, be sure to trim it often — every two months — to avoid a stringy look, says Lisa Fiorentino, a New York stylist. And use a thickening spritz to pump up the individual hairs, giving the illusion of a fuller mane.

Have thick hair? Opt for all-over layers so you don’t end up with a pyramid look, à la Rosanne Rosanna-Danna. Curly and wavy locks also look great with lots of long layers, but they demand extra care. Nurture them with a deep-conditioning treatment once a week.