It kinda depends on what's bringing you down. Getting your hair, nails or a new outfit always helps but won't keep your spirit up. Finding something that makes you happy and getting rid of the things that don't is what really helps.
As far as physical things go, what really increase my self confidence would be wearing heels (sometimes), having matching lacy underwear, perfectly curled hair, red lipstick and even eyeliner. But seriously just applying red lipstick when I wear makeup evokes something in me that helps with any negative thoughts :3 Tea and uplifting music also can really help. If you can, try doing something you enjoy (perhaps wearing your makeup like that) outdoors.