MARIO DIAB treats his clients' dry, stressed tresses with the Leonor Greyl four-step Cult Hair Spa Treatment at his namesake salon in Manhattan. First, he applies a bit of Huile de Palme oil with his hands. Next, he massages a few drops of Regenerescence Naturelle—full of essential oils—to stimulate blood circulation in the scalp, then uses a paintbrush to apply Crème aux Fleurs, a creamy washing treatment for nourishing dry, color-treated or frizzy hair. Finally, he applies one of three masks—Masque Fleurs de Jasmin, Masque à l'Orchidée or Crème Regeneratrice—and allows it to penetrate for 15 minutes before rinsing. Voilà! Hair is shiny, soft and manageable. —L.A.