silver bronze copper

Put a huge piece of aluminum foil in a sink so it covers it with water that is very hot. The water should be lower than the fill line of the sink so there is room to place your item inside. Add about a half a cup of salt and stir it all around in the water to dissolve the salt into the water. Place your silver item in the water and let it sink to the bottom so it is completely covered. Allow it to soak in the bath for about five minutes. Rinse with warm clean water and set to dry. Do this regularly to keep your silver tarnish free.
Another way to be rid of tarnish, is to sprinkle some baking soda on a damp sponge and rub until clean. Then just rinse and dry.
For tarnish that is stubborn as a bull, make a classic paste of baking soda and water and cover the silver with that. Let it dry then rinse the paste off with warm water. Use a toothbrush for those difficult to reach areas.

brass
Cleaning Brass Naturally

Start out by mixing baking soda and fresh lemon juice until it is a paste-like consistency. Then, rub the mixture all over the brass surface and allow the paste to sit on the brass for about five minutes or so. After the five minutes rinse the mixture off with warm water.
Brightening, the brass, after polishing, pour a little bit of olive oil on a soft cloth and rub the olive oil on the brass. Be sure it is a thin enough layer as to not make it slippery.

copper

Cleaning Copper Naturally

Get a spray bottle.  Pour distilled white vinegar into the spray bottle til it is able to spray with no issues. Spray, so there is a beautiful layer on the copper. Then, sprinkle salt onto the vinegar layer and rub a soft cloth all over it. Rinse everything off with warm water. Copper that is ridiculously oxidized the item will probably need one or more treatments for full shine.