Always remove your jewelry before swimming in the sea or the swimming pool, and prior to showering. Also, remove when doing household cleaning to avoid contact with strong chemicals. Avoid getting your jewelry in contact with beauty products and perfume.
When not wearing store your jewelry in ziplock bags to prevent tarnish.
If your jewelry needs cleaning gently rub with baking soda and warm water and then dry with a soft cloth. I do not recommend using a Silver Cleaner.
The black oxidized layer will fade with everyday or very frequent wear revealing the natural color of silver beneath. 24K gold plating, also. Both surface treatments can be renewed at a low cost.