I found this advice here
http://www.preservation.gc.ca/howto/articles/brass-copper_e.asp
"A mild polish, such as jeweller's cloth, is recommended for most mildly tarnished copper, brass and bronze pieces. The cloth contains a fine abrasive, but even so excess metal can be removed by rubbing too hard. Use a more abrasive polish, such as liquid metal polish, only if using the cloth is ineffective. Most polishes leave a blackish residue, which can be removed by softly polishing with a plain piece of flannelette.
Polishing should be done only if the surface can be protected from tarnishing again. There is no point in polishing away material from the surface if tarnish is allowed to reform. Wax coatings offer some protection and can reduce the need for frequent cleaning or polishing. A suitable coating can be made by combining equal parts mineral spirits (for example, Shellsol or Varsol) with a bleached paste wax such as Renaissance wax or a high-quality, white furniture wax. The mixture should be stored in a tightly sealed jar.
To apply, wipe or brush the wax mixture over the object. Then, set it aside so the solvent can evaporate. Be careful to apply it evenly. If the article does not contain wood, ivory or other heat-sensitive material, a hair dryer can be used to melt the wax, ensuring that it gets into recessed areas. Blot away excess wax with tissues while it is still warm. Once the wax sets, buff the object with a clean, lint-free soft cloth.
To remove the wax, use mineral spirits. Be sure to do this only in a well-ventilated room and wear rubber gloves to protect your skin. Small amounts of leftover solvent (or solvent-saturated cleaning cloths) can be disposed of by letting them evaporate until they are dry in a well-ventilated area. Then, put the solid residues in plastic bags and dispose of them.
An application of lacquer is sometimes advisable for objects that will be handled regularly or are exposed to high humidity, pollutants, etc. It is advisable to have this done by a specialist because a poorly applied coating can be disfiguring and does not provide adequate protection."