Smiling, which usually feels good, can also make you feel self-conscious if your teeth are less than white or your lips are lined or cracked. But a slew of new treatments and products can keep you from wanting to stifle that grin. Here are the most effective teeth whiteners and brighteners, plus tips on selecting and applying gorgeous and goof-proof shades of red lipstick — just in time for little-black-dress season. Get whiter teeth at home Strips, trays, toothpastes...what really works? Here, our experts reveal their favorites, along with advice on who should see a dentist before considering the DIY approach. Why do teeth change color? Blame time and your diet. With age, the enamel on teeth becomes thinner and more transparent, and the inner layer, called dentin, looks darker. Teeth also absorb colored liquids throughout your life, says Jeff Golub-Evans, D.D.S., a cosmetic dentist in New York City. Coffee, tea, cola, and red wine are frequent culprits, along with more tenacious tobacco stains. How do at-home whiteners work? Peroxide is the key ingredient in most whiteners. This safe-for-the-mouth bleaching agent forms bubbles on enamel that lift away stains. The higher the concentration of peroxide and the longer you leave it on your teeth, the whiter they'll get. The downside: Bleaching molecules can get trapped in nerve passageways, causing increased, though temporary, tooth sensitivity. Who's a good candidate? "At-home whitening is an option for anyone with a healthy mouth who has been to the dentist in the past year," says Matthew Messina, D.D.S., a consumer adviser for the American Dental Association who practices in Cleveland. Three groups shouldn't do their own whitening: those whose teeth are painfully sensitive to cold; anyone with crowns or fillings on their front teeth (they won't whiten and will end up looking much darker than surrounding teeth); and people whose enamel seems more gray than yellow (due to intrinsic stains from antibiotics like tetracycline taken in childhood). Assuming your dentist has given you the OK, Dr. Golub-Evans says you can assess your whitening potential this way: Hold a piece of white printer paper next to your teeth. If they look yellow, the stains are probably just on the surface — teeth should turn at least a couple of shades lighter with at-home bleaching. If your teeth look grayish, the discoloration likely lies inside the teeth, and bleaching won't help much. Which products work best to brighten your smile Trays and strips are the most effective at-home choices for dramatic whitening, say dentists. These "barrier method" whiteners keep the peroxide solution against the teeth for the longest time. Daily use can whiten teeth five or more shades. Most brands of trays and strips recommend you use them for around 30 minutes a day until you get to the desired shade (usually at least a week). For the most dazzling results, look for higher concentrations of peroxide combined with longer application times. Crest Whitestrips Premium ($35, drugstores) contains 10 percent peroxide and calls for wearing the strips for 30 minutes twice a day for a week. The fit of the trays or strips stops some people from using them. Unlike a custom tray from the dentist's office, disposable versions are made with generic molds designed for the average-size mouth. They can leak peroxide solution through any gaps and cause tooth and gum irritation. Whitening strips easily wrap around teeth but sometimes slip and slide. (In early 2009, Crest will introduce a new whitening strip it claims will have better hold.) To reduce potential gum irritation with both trays and strips, don't brush your teeth for up to half an hour before applying them — or at least skip the toothpaste, suggests Robert Gerlach, D.D.S., a research fellow at P&G and developer of the original Crest Whitestrips. "Detergents in toothpaste can make gums more susceptible to irritation," he says. Paint-on whiteners let you target the peroxide solution to specific teeth and spot-treat discolored cracks and chips on enamel. But because there's nothing holding the peroxide to the tooth, saliva quickly dilutes it. Some versions employ the same technology that gives long-lasting lipsticks more hold. "The whitening solution forms a film that protects the peroxide," explains Dr. Golub-Evans. The Good Housekeeping Research Institute compared the performances of three paint-on teeth whiteners to a leading over-the-counter whitener and found that if you have sensitive teeth, the paint-on whiteners were the better choice. Testers used the products for either one or two weeks, according to the package directions. They gave GoSmile Advanced Formula B1 high marks for its simplicity (pull out a pellet, snap it, and apply) and because, in most cases, it caused the least tooth sensitivity. "I liked the easy use [of GoSmile]," said one woman. "It was portable and not messy." Yet, while our volunteers thought GoSmile whitened effectively, it was outperformed by the control product in our laboratory before-and-after shade evaluations. Toothpastes and toothbrushes brighten teeth a shade or two at most. A toothbrush featuring an oscillating motion will give you better results than a manual toothbrush, says Dr. Gerlach. But you have to brush for the recommended two minutes. As for whitening toothpastes, the most effective ones include ingredients that prevent new stains from sticking to teeth, like Colgate Total Advanced Whitening. Mouthwash, the latest tooth brightener on the market, can swish away only the most superficial stains, since it doesn't stay in contact with teeth for very long. (In fact, if you don't use tobacco or drink coffee, tea, colas, or red wine, you may not need it.) Try Listerine Whitening Vibrant White Pre-Brush Rinse. What more can a dentist do? They make custom bleaching trays for at-home use, and they can apply a masking gel or dam to protect gums during stronger, in-office treatments. Many dentists have accelerated bleaching options that get teeth up to 10 shades lighter in a single visit. Zoom! and BriteSmile are the two most popular. These use light to pump up the effects of double-digit-strength peroxide. However, any dramatic whitening can fade quickly without at-home maintenance (usually a tray, whitening strips, or toothpaste). You also have to watch what you eat and drink for a couple of days, since tooth enamel is very receptive to new color for the first 48 hours after a bleaching treatment. Makeup tricks to make teeth look more white 3 tips for great lips • Wipe out flakes. Smooth lips by gently rubbing them with a damp washcloth or soft toothbrush, then apply lip balm, suggests Jo Strettell, makeup artist for Rimmel. If the rough patches persist, it may actually be a reaction to the fragrance in your lip balm or lipstick. "Chronically peeling lips are often a sign of an allergy," says Santa Monica dermatologist Ava Shamban, M.D., who suggests switching to a hypoallergenic product. • Disguise lip lines. Exfoliation and hydration are also key to smoothing lip lines. Try a thick balm or hydrating treatment like Le Couvent des Minimes Nutritive Lip Balm. Then slick on a shiny gloss or lipstick; matte formulas can get caught in the creases. • Fake fullness. Lip-plumping glosses are a fun way to fill out thin lips. They rely on ingredients — like cinnamon — that temporarily swell the skin. For best results, apply them to a bare mouth. "There's no downside as long as you're not allergic to the plumping ingredient," says Dr. Shamban. You can also opt for a gloss with shimmer to give lips a more 3-D look. Red lipstick looks festive — and can make your teeth look brighter. Here's help in finding (and putting on) the right red hue for you. • Use this foolproof method: Choose a color based on your undertones, says L.A. makeup artist Heather Currie. If your skin is olive, orangey reds look best. If pink undertones predominate, try blue-based reds. True reds — neither too blue nor too orange — are the tooth brighteners. They look good on everyone. • Dress up lips with berry tones if you're red-shy; they're slightly less siren-y. Try a wine or plum shade. • Opt for sheer formulas if your skin is blotchy or uneven. Conversely, the deepest, most opaque reds flatter smooth, even skin. • Match your liner to red lipstick. You want reds to look really perfect, suggests celebrity makeup artist Joanna Schlip. • Keep lipstick off teeth with this Hollywood makeup artist's move: Apply your shade, then place a clean finger in your mouth, purse your lips, and pull your finger out along with any excess color. Lasting tip Give your favorite shade more staying power by filling in lips with liner, placing color over it, then dabbing on translucent powder. Add another coat of color and a second light dusting of powder. Or try a long-lasting lipstick; the latest formulas don't dry out lips Five quick ways to whiten your teeth In the shower: Stop stains and tartar buildup in five minutes a day. Crest Whitestrips Daily Whitening Plus Tartar Protection. At snack time: Choose crunchy fruits and veggies, such as apple slices, carrots, and celery sticks. They actually help clean your teeth while you munch, explains cosmetic dentist Jeff Golub-Evans, D.D.S. After lunch: Chewing sugar-free gum can absorb some of the new-to-the-surface stains when you don't have time to brush. Supersmile Professional Whitening Gum Night out: This gloss's blue undertones make teeth appear whiter; its shiny, minty formula gives lips, and breath, a boost. CoverGirl Wetslicks AmazeMint in Plum Crazy. After dinner: Swish water in your mouth for 30 seconds right after eating to help prevent stains, says Dr. Golub-Evans.