From a Southwest Airlines employee: "Welcome aboard Southwest Flight XXX to Chicago. To operate your seatbelt, insert the metal tab into the buckle, and pull tight. It works just like every other seatbelt and if you don't know how to operate one, you probably shouldn't be out in public unsupervised. In the event of a sudden loss of cabin pressure, oxygen masks will descend from the ceiling. Stop screaming, grab the mask, and pull it over your face. If you have a small child traveling with you, secure your mask before assisting with theirs. If you are traveling with two small children, decide now which one you love more."
Southwest often has the best ones, I can't remember the full details of the one for a redeye ISP->ORD but it was quite funny, along the lines of "you've heard this before and aren't paying attention so we make it super funny"