Problem: Women tend to hang on to their makeup for much longer than they should. Flaking is a good sign that your mascara is either not a good formula or needs to be replaced -- whether you've finished the tube or not.

Solution: If you find that your mascara runs and smudges under your eyes during the day, try a waterproof formula (like CoverGirl LashBlast Waterproof Mascara, $7.99) or one of those "tubing" mascaras (like Clinique Lash Power Mascara, $14). Tip: Waterproof mascara can be difficult to get off so one solution is to use your regular mascara and then just use a layer of a waterproof formula over it, or just use it on the bottom lashes.

Problem: "The thing that bothers me the most about glitter in makeup, besides the fact that it shouldn't be worn by anyone over the age of 14 (fabulous club kids get an exception here) is that it travels on your face ... leaving strange sparkly spots where they shouldn't be," says Stiles.

Solution: If you have a sparkly personality and must express it through your makeup, a more grown-up option would be to ditch shadows that contain large flecks of in-your-face glitter for those with very, very finely milled luminescence or shimmer (like L'Oreal HiP Shocking Shadow Pigments, $12). And always apply with a restrained hand. (See mistake No. 9 for how-to.)

Problem: Too much bronzer can make your face look dirty or too orange, or too shimmery if you're using a bronzer that isn't matte.

Solution: "I do like a bronzer with a little sheen to it, just be careful where you apply it or you'll be guilty of mistake No. 9," says Stiles. "There are a few companies that make lovely matte bronzers that look completely realistic. Benefit Hoola, $28, is always in my kit. Physicians Formula is almost entirely made up of bronzers." The color of your bronzer, like foundations, should change throughout the year as well. A little bronzer in the winter can add a touch of warmth without making you look too tan.

Tip: Guerlain Terracotta Spray Bronzing Powder Mist, $57, is a foundation in a spray that offers a very realistic tan color. Spray it in your hand then apply with a flat foundation brush to cover tan lines or add a touch of color on your forehead and cheekbones to warm up skin without it looking like makeup.

Problem: "Luminous particles have been added to everything in the past few years, most notably foundations, powders and highlighting products," says Stiles. "One or two [such] products can make a woman look fresh and glowing. Too many can make her look very, very strange." You run the risk of your face texture not matching that of your body.

Solution: There are a few places where luminizing products work: the cheekbones, the inside corners of the eye, and if the products are subtle enough, under the eye. (Concealers like YSL Touche Eclat, $40, that have a hint of luminescence can really combat dark circles.) Avoid using a luminous foundation with a shimmer powder on your cheekbones and a very shimmery eye shadow. "What looks good in magazines doesn't always translate well to real life, so approach with a restrained hand," says Stiles.

Problem: Good makeup starts with a good canvas, which is hard to achieve if you're not taking care of your skin. "I cannot express the importance of a good skin care regime," says Stiles, echoing many makeup artists who cite poor habits (i.e. smoking and staying up late) and lack of sun protection and moisture as main contributors to bad skin.

Solution: Don't just use any product. See a facialist and do some research to find a good cleanser, moisturizer and targeted treatment products that are suited to your skin type. Also, consult our guide to finding The Right Skin Care for Your Age. And don't forget to protect your skin daily with a high-SPF sunblock like Ocean Potion Face Potion, $3.99, which "isn't greasy, doesn't leave a white cast, and costs under $5," says Stiles.

"Remember, your face ends at your nipples," says Stiles. "Don't neglect your neck and chest; they need sunblock and moisturizer too." (Your chest gets so much sun during driving and walking time, you may not even realize it.) Find out what else you can be doing to protect your skin daily here.