Problem: You may not be able to see that your foundation ends in a line at your jaw, but everyone else can. Blending is the key to a flawless face. If your blush sits unblended on your cheeks or the little eye shadow you do wear is just plopped on your lid, it's going to look like you're wearing more makeup than you really are.

Solution: Only wear foundation where you need it and only blend past your jawline if your face doesn't match your neck. Other than that, the key is to get the right tools. "A few good brushes will make the difference between makeup and blended makeup," says Stiles. (Try the MAC 190 Foundation Brush, $32, and Sonia Kashuk Tools Powder Brush, $19.99 at Target.) Latex wedges (like the beautyblender, $19.95) are also good for blending foundations and blushes as you can easily dilute color with a few swipes if you've applied too much.

Problem: This happens to every woman. Even if you've found the perfect color for your skin, the foundation you buy in August will not work for you in February. Also, watch out for foundation that oxidizes throughout the day and changes color.

Solution: Every woman should have at least two different colors of foundation, but three is better. That way you always have colors to mix to get the right color for your skin year-round. Focus on finding a good formula that works with your skin tone and doesn't turn orange (from oxidation) as it interacts with the oils in your skin. (Keep in mind that your face should look like it matches your body. Sheer formulas are better if your face is paler than your chest. Tinted moisturizers are terrific if you don't need much coverage.) The same holds true for concealers. (Tip: Use under-eye concealers that have a subtle light reflective quality and use matte concealers to hide spots.) Also, using a truly translucent powder (like Physicians Formula Organic Wear 100% Natural Origin Loose Powder, $13.95) when you set your foundation (and for touch-ups during the day) will help combat a shift in color.

Problem: A full-on eye with a full-on lip with a full-on face of foundation, blush, bronzer and more is only going to make you look older.

Solution: You don't need to focus equal attention on every feature at the same time. Whether you love experimenting with makeup or not, pick one feature and accentuate it. For example, pair a bright lip color like fire engine red or electric pink with a fresh, natural face -- not too much foundation, blush or eye makeup like Denise Richards, right. Focusing color on the eyes? Go easy on the mouth with a pale, sheer lip color. "It will make the strong eye look modern and more interesting," says Stiles. "Anything thick or heavy will look aging."

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.)