I have a Samsung P-3 and the touch screen gets frequently grimy, using common sense and what I obtained through the Samsung Wikipedia shell (Samsungwiki i think its called, Google it!) I just apply a small amount of Windex/window cleaner (P3 screen is glass so its natural) and almost dry brush the screen with windex, then rub it off with the other side of the paper towl, no biggie, it even makes any scratches you could have on it invisible until they get more dirt in them. I also have a leather case with a mini pocket on the cover flap where i keep the same cloth used to clean eyeglasses, just remove the case, breath on the screen and wipe, works like a charm.
Hope this helps somebody. !
P.S don't do what I do and eat pizza then directly want to listen to your favorite song....grease does not go well on leather or glass ;P