Tarnishing, as in a darkening of metals such as silver, nickel, copper, brass, etc., is caused by impurities in the metal oxidizing in the air. The only remedy is to keep your jewelry polished or buffed.
There are products, like Silver Protector Strips, made by 3M, that you can keep in your jewelry cases which absorbs moisture from the air…much the same as the little packets of silica gel crystals that are often included with new shoes.
If we are talking about a base metal, covered in either an acrylic metallic color, such as a bright gold foil appearance, the only solution is to avoid contact with oils, salts and acids.
Since human skin transfers all of these, you can either wipe the jewelry off with a soft cloth when you remove it, or look into applying a clear, high gloss water-based varnish, available at craft stores.
This is not a permanent solution, but keeping the pieces clean and away from contaminants is the simplest way to preserve the appearance. Avoid oil-based varnishes and shellacs, as they tend to yellow, and crack.