Sterling silver naturally darkens and tarnishes over time due to exposure to air and the elements. Contact with salty air, prolonged moisture, chlorine, household chemicals or cosmetics will accelerate this process.
Tarnish can be removed with a soft silver polishing cloth.
If the piece is a bit worse for wear you can use a soft toothbrush, some warm water and a mild dish washing detergent. Very gently give the piece a soak and scrub and then dry with a soft cloth. Then give a light polish with a polishing cloth.
To keep items in their best condition it is recommended to remove pieces before applying perfumes or cosmetics, as well as while sleeping, showering, exercising, swimming or cleaning with harsh chemicals. Store items in a cool dry place, separate to other jewellery items as they can scratch one another.