I recommend removing necklaces when showering or swimming because water and chemicals (especially chlorine) can affect the longevity of the gold plating, encourage silver to tarnish and dull gemstones. Always apply make-up, hair spray, perfume, moisturiser etc before putting your jewellery on as the chemicals in these can affect the jewellery.
To clean your jewellery and restore its shine, use a clean, soft and dry 100% cotton or microfibre cloth to gently wipe the surface. Do not use a polishing agent or abrasive compounds as using these will wear away the layers of gold. Take care to store your jewellery separately so that it does not get damaged or scratched by other pieces. If possible store your jewellery in its original box, to help prevent tarnishing.