We've had this thread before, but I've since done a little experimenting. I bought some tarnished pins at the outlet last time I was in Orangefair. I used some white vinegar to clean the tarnish and oily finger prints. They got off most of the tarnish, but not all of the finger prints. Anyway, I think that's a good thing to use if you have tarnish on the metal (apply with a cotton swab). Otherwise a wipe with a damp (not wet) paper towel and mild dish detergent should do the trick.
As well, just keeping your pins in plastic bags and not handling them directly can facilitate keeping them clean and detering tarnishing or scratching.