Using skin care products to look good on the outside may not be enough if you feel tired on the inside. Global direct seller Nu Skin Enterprises (NUS) bets its business on that notion: It sells products that aim to deliver improvements on the inside and out. After focusing most of its new products on skin care the last few years, it's now in the midst of revving up the nutrition side. The current vehicle of choice is called Vitality, touted as an