It is important to know that brass, like other metals and leather, begins to take on a new life with its wearer and change depending on how it is cared for and used. It's a metal that is pretty easy to take care of, keeping it dry and away from moisturizers or chemicals.
You can easily polish brass with lemon juice on a clean cloth. You may also mix the lemon juice with baking soda for more intense results. Personally wearing my brass bracelets everyday, I've found that spots will come and go without polishing.
If you are seeing dark spots appear on your piece, there is probably residue from a moisturizer or soap on the brass, in which case you may want to gently scrape it off with your fingernail or rub with a dry cloth and see if the brass clears up over time. If you definitely prefer your brass shiny, try the lemon juice!
Feel free to contact me with any questions.