Always keep jewelry clean and dry for best results. Store pieces of jewelry individually to avoid scratching and remove before you swim/shower/sleep. Wait until any lotion/hairspray/product you apply has dried before you put on your jewelry.
Brass & silver:
These metals will naturally tarnish over time and the oxidization will accelerate when exposed to more oxygen, moisture or natural oils. Keep the jewelry clean and dry and polish occasionally with a soft jewelry cloth.
Take care of your gold-plated jewelry and it will take care of you! The fine gold coating can wear off over time, especially with heavy use, so take care to keep it from getting scratched. It can be cleaned with a damp soft cloth and can be re-plated eventually if necessary.