Your polymer clay earrings are durable, however they can become damaged so it is reccomended to store them in a cool, dry place away from sunlight.
All jewellery that has any metal components such brass can tarnish over time, this is a normal thing with metals. You can clean them up by polishing with a jewellery cloth. Brass cleans up with water, baking soda and a small amount of lemon mixed. Use a cotton bud or ball, and gently wipe over the brass components. Dry with a cloth.
If you find over time the clay is becoming dull, you can refresh your earrings with a small amount of baby oil and run over with a cotton bud. If your earrings are attracting lint, run a small amount of nail polish remover or alcohol with a cotton bud over the earrings.