While you're wearing your jewelry, keep it away from moisture (e.g. sweat, lotion, rain.) In between wearings, do as we do at the shop: store jewelry in a plastic bag and use a soft cloth to gently buff anything that looks a little dull.
14k Goldfill is a thick layer of 14k gold bonded to a base metal. It's tarnish resistant, and doesn't flake off or change color. If you can wear 14k gold, you can wear 14k goldfill.
Sterling Silver is an alloy of silver and a tiny bit of copper. If it starts to look dull or yellow, give it a few swipes with a jewelry polish cloth to bring back its gleam.
Brass is an alloy of copper and zinc. Brass jewelry will darken and acquire its own unique patina over time. You can, of course, embrace the aged look of your piece, or brighten it up with a polish cloth.
Oxidized Brass is intentionally blackened and will lose its finish if it's polished. If the finish does start to come off, don't worry! Brass can be re-oxidized by a jeweler if necessary.