(I haven't tried the following, but if you do, you do so at your own risk!)
You could try some rubbing alcohol (on one). It should cut through the fingerprint oils and evaporate off, but how you get off any 'spotting' from the residue is beyond me, unless you blot it with something (non-scratching, of course) like a cotton swab.
And once you get them clean, better pop'em into plastic coin holders...!