If you're old enough to have a margarita, you'll want to read this, since skin gets duller starting around age 20, when cell turnover slows. And if you had a margarita last night, you'll really want to read this, since alcohol, sugar, and salt all zap radiance (sorry!): Using a serum or cream with brightening ingredients, like vitamin C or soy, each morning can make your skin radiant in about one month (we like Aveeno Positively Radiant Daily Moisturizer SPF 30 with soy and SkinCeuticals C E Ferulic Combination Antioxidant Treatment with vitamin C). If your skin is sensitive, try a formula with niacinamide instead, which calms redness while brightening (we like Olay Regenerist Luminous Tone Perfecting Cream Moisturizer). Plus, all three brightening ingredients help fade brown spots that prevent skin from reflecting light and prevent new ones. "By 30, many people have pigmentation under the skin that's not yet visible to the naked eye," says Zoe Draelos, a consulting professor of dermatology at Duke University. "Treating spots before you see them can keep them from surfacing.