My DS came out with "f*** sake" after hearing it once from me in sheer frustration (I was severely provoked). And he used it in context...eg when he dropped a toy, when I didn't do something the way he wanted me to do it. We started off ignoring it and really making sure that we were never dropping the F bomb ourselves (we aren't swearing people but we're not perfect) but that didn't stop him.

So we started saying in a very matter of fact way, no jumping up and down "no, we don't say that, we say goodness sake" and if he said it we would just emphasise "goodness sake" and within a week we never heard it again...except for once 2 days ago and he immediately corrected himself and said "goodness sake". And haven't heard it again.