Hi Jane,
Medals are not one of my interests, but
as an antiques collector/dealer, I have seen too many things
ruined by well-meaning people, armed with household cleaning products and polishes. Some metals such as bronze
should never be polished. I don't know what you mean by "dirty". If they are in really bad condition such as going green from being stored in a damp place, it is a job best left to professionals. If they are not that bad, but you feel that you really must clean them, be very careful. Soap and water, as Flipster suggested, should be ok, but make sure that they are really dry afterwards(a short time in the oven should do it). Do not use any abrasive
cleaners or polishes.
I found a website which gives advice on cleaning medals and ribbons. I just took a quick look and products recommended are ones that I have used on other metal
objects and found to be satisfactory.
http://www.awm.gov.au/aboutus/conservation/medals.htm
Cheers,
Carmela