When different metals are exposed to each other, especially in the presence of moisture, they can quickly tarnish and lose their shine. Therefore, you'll want to stop the problem as soon as you spot it.
Clean slightly tarnished brass or copper with half a lemon or lime dipped in salt. The salt works the acid from the lemons or limes deep into the tarnish and scrubs away the bits of dirt and grime. Consider it an exfoliating facial for your copper and silver.