A basic rule of thumb is to avoid unnecessary light. If your're not using your jewelry just store it in a box, away from humidity! But why should you even think about that, your'e always wearing your jewelry anyways ;) ! Also, avoid water at all cost unless your planning on drying it right away, better safe than wet!
Avoid chemicals as perfume or cream directly on the jewelry.
Certain stones are more fragile than others, like moonstones and Turquoises. Moonstone is a soft stone and crack easily. To avoid cracking, don't do physical work with your jewelry, it is better to remove it and store it. For turquoise stone, it can discoloured with high heat or contact with chemicals (cleaning products, perfumes, etc) It is safe to clean both in soapy warm water!
Treat it as paper, because it is made of it. Paper hates water so does your beautiful earrings! Ideally stored in a dry place and it's original box to keep it's shape (not in the bottom of a purse ;) ). But again, we don't want to see those bad boys crying all alone on your dresser, we want to see them dangling on your earlobes!
Sterling silver tarnishes. It will come to a point that maybe you'll want that mirror shiny look again and that, you can do my friend! For the real dirty stuff, do a first clean with a cotton ball (anything cotton-based) and some hand sanitizer, get that dirt off! You can also use a polishing cloth or a gentle silver polishing cream using a sponge. Once that's done, make sure you rinse that cream off with warm water (it's easier to evaporate) and dry it right away with a dry towel!
-Source, Rio Grande
Brass metal tarnishes faster than sterling silver, that is why it needs extra care. To remove the tarnish, make a paste with baking soda and lemon juice, scrub the jewelry with your fingers. Let it sit for 10 minutes.
Another easy way is to put the jewelry in Ketchup!!! Yes ketchup, with a bit of water, let it sit for 20 min and rinse with warm soapy water and dry well.