Mireille Guiliano has already had her first Nicole Kidman moment. Last week, while she was traveling to Chicago to promote her runaway best-seller "French Women Don't Get Fat," the ticket agent recognized her. Loudly. "I was so embarrassed, I didn't know where to look," says Guiliano, laughing. She'd better get used to it. Her book--part memoir, part cookbook, part how-to--peppered throughout with French phrases (naturellement), has been in stores less than a month, and already it's in its sixth printing and has sold close to 300,000 copies. Guiliano, who at 58 weighs 110 pounds, and goes to hundreds of fancy restaurants a year as the CEO of the U.S.-based Champagne Veuve Clicquot, says she never puts on weight. Her common-sense "secrets"? Dine for pleasure, drink lots of water, eat small portions--and yogurt. And, whatever you do, don't think of the regime as "a diet." "It's a boring word, a misleading word, a depressing word. It's so negative, and I think it stresses people enormously," she says. Never mind that she recommends kick-starting her program with a weekend devoted to eating nothing but her "magical leek soup." One fan has already written to her predicting a world shortage of the oniony vegetable because of the multitudes who are following her advice. Unfortunately, the fans will have to wait. Guiliano is about to jet off to Australia, where she intends to preach that French women love chocolate--but only a petite square.