Scalp buildup -- think oil, hair products and dirt -- can contribute to thinning hair. And Bumble and Bumble Full Potential Hair Preserving Shampoo, $31, directly addresses these causes by exfoliating the scalp. It contains menthol, which tingles while it cleans, as well as creatine to reinforce hair strands and UV absorbers to protect the scalp and hair from sun damage.