3. Cold showers
Business consultant Nate Halsey, writes Stone, takes an invigorating cold shower every morning, a habit he acquired from a German friend who had learned the behavior in boarding school—and claimed it was the reason he never got sick. Halsey credits his daily cold shower not just for his good health, but also for preventing dry skin. Stone also details several small European studies that have found immunity benefits to cold showers and baths (as well as in rolling in the snow after the sauna).