Due to its nature, silvers darken by time because of the sulphur in the nature. but its in your hands to prevent this. In order to cleaning it, keep your silver in a closed space such as its own box while you dont use it and if you wipe it with the fabric before you put, you will see it will get darker much slower.
Another way is using some thoothpaste. although this method seems very unconventional, you will be surprised of the result. First put some thoothpaste onto silver and put it in a cup of water. after waiting half an hour, take it out and dry. Da-daam, its shining…
The same way as thoothpaste method, you can use some vinegar and baking powder. 2 spoon of of vinegar and some powder will be just enough to polish your silver.
Another option is lemon and salt. Easily add some lemon and salt to the water and apply the same procedure.
One last way: Put some water in a bowl, add a piece of aliminium and some salt to water. put the ring in this water at least for ten seconds and take it out