Although it is normal for copper to lose its shine over time, it is possible to keep it shiny by cleaning it periodically. An effective method is to use salt and lemon: pour a little coarse salt into half a lemon and then rub the surface of the copper. This slightly abrasive cleaning will revive the shine of all copper items. Then rinse with warm water and dry with a soft cloth.
An alternative is to rub copper objects with a mixture of flour, coarse salt and vinegar. Then rinse with a microfiber cloth.

You can finally clean copper with ketchup! The vinegar in ketchup can work wonders. Apply a thick layer of ketchup on the copper object to be cleaned, let it sit for a few minutes and then rinse with hot water.