I'd like to step in here, as I work in conservation of museum objects (not metals), but collect badges, and have access to conservators and conservation scientists. One of my colleagues, who specialises in metals research advises against the use of any commercial products, since they tend to contain either ammonia or a weak acid (acetic acid). All of these products will cause corrosion and/or pitting. In some cases, with brass, they will actually strip off the zinc, what is referred to as "dezincification", leaving the copper underneath. Then you are left with a pinkish colour. As my colleague points out in response to a friend of mine asking about the cleaning of a badge that had been dug up and had all sorts of crud on it,
"Brass is an alloy of copper and zinc. The pink colour on the brass badge is probably from dezincification. Dezincification means loss of zinc. It is possible that the dezincification happened during burial when the corrosion process resulted in the selective removal of the zinc which leaves behind copper. Relatively pure copper is pink - like the colour of a new penny. It is also possible that the dezincification has been caused during cleaning if the brass was cleaned with an acid (usually a weak acid). The acid can selectively dissolve the zinc from the surface.
I do not know how to reverse the process of dezincification.
Calcium deposits can be removed with acids, but in this case, I would not recommend using an acid, given that you could further damage the brass by dissolving even more zinc."
The only safe product recommended is precipitated chalk. There is an article on the US National Park Service website on Caring for Silver and Copper Alloy Objects which might be of interest to everyone:
http://www.nps.gov/history/museum/pu.../cons_toc.html
Scroll down to Metals and click on 10/2. All the articles are in pdf.
Of course, it goes without saying, that anodised aluminum (or aluminium to you Brits), otherwise known as 'Staybrite', should never be cleaned.
If you have access to a conservator at your local museum, you should contact them for further advice.
I would agree with other here on the site, that unless it's absolutely necessary, I would leave them alone. It removes the patina and destroys the overall integrity of the item.