The golden rule of sunscreen? You’ve got to apply it often and amply for the stuff to really do its thing. The American Academy of Dermatology recommends smearing on one ounce — enough to fill a shot glass — to cover your exposed areas. But the majority of people only use 25-50% of the suggested amount. With many formulas topping $40 a bottle, it’s no wonder why: Who can afford a $10-a-day SPF habit?