As you age the grip of your finger prints lessen as the ridges dissappear (fail to form). Ask anyone who has ever been taxed with getting a viable finger-print from someone who is over 50. The condition can be quite profound in some people, such that they have virtually no fingerprints to speak of. That is the real reason why people drop things more as they age: the amount of pressure that a lifetime of experience has advised them to use, given a certain tensile tackiness to their fingers, is no longer sufficient to stop an item of a given weight from following gravity to the floor.
I have this problem when I play classical pieces that require straight fingers because the spread needed to accomodate non-conventional chords and arpeggios in 'black-key' tonal centers causes the finger to adopt a more acute angle of approach on the keys in the center of any given hand position; more so the third finger, rather than any thumb cross-over point. Debussy's, Claire De Lune, is just such a piece. It's not fast or hard, it just invites this kind of key slide and resulting error; partly because a flat hand is used to secure the desired articulation, tone and effect, making things worse.
Any fine abrassive polish like brasso will lightly scourge the shine from plastic keys and retain the dust as a paste to be easily wiped of, and any polish polish will restore it if you change your mind. If you have a mechanical piano, fine quality (like 3M) wet/dry automotive paper will cause less residue (and it is dry, powdery and vacuumable) than either steel-wool or scouring pads, and that way you can use a sanding block to do 2 or 3 key tops at a time to cause uniform striations (or no visible etching in the case where very fine grade has been used). It will only take a few passes, and if you don't like it, polish it again with either McGuires or Mothers Plastic Polish.
Additionally, I just tried a Mr. Clean Magic Eraser, which worked very well too. Get creative!
