It's as plain as the wrinkles on your face: New York is tough on your skin.   New Yorkers are the most wrinkled people in the country, according to a recent study. And like other Americans, we increasingly take advantage of available cosmetic procedures.   "Even though we don't think of New York as being one of these super-sunny places, we spend a lot of time walking outside," says Dr. Sachin Shridharani, a Fifth Ave. plastic surgeon. "And even though we are not out there with the aluminum foil (reflector) catching the rays, we don't think about how much exposure we get to U.V."  Read more...