In an attempt to protect my keyboard, I bought one of those expensive silicon covers that you place over your keyboard. Later I found that the oil from my fingers still seeps through the cover and got locked in there because, now, the cover was stopping the oil from getting wiped away.
So I just took it off and replaced it with 3M magic tape. Buy a one-inch and half-inch roll, cut it into squares and tape it over your keys. This actually worked better for me because the fit is tighter and the tape is cheaper (and you could use the remaining tape to wrap gifts and books) :)