Big dosages of biotin. My mother has more or less destroyed her hair over decades of nutrient-poor diet, bleaching it and unmanaged stress. She is in her 50s now and her hair had become very thin some years ago. She started taking lots of biotin and it has actually made a noticeable difference. Which still shocks me because I tend to be pretty skeptical about such things. She doesn't have thick, luscious locks or anything, but her hair is normal density again.