Three announcements I've made in the past on that issue...
Ladies & Gentlemen, Please remain seated as impromptu visits to the flight deck are not allowed, and the captain has big feet for the little brake pedals.
Ladies and gentlemen, unless you have a superhuman ability to leave the aircraft before the door is open, we ask you kindly to remain seated.
Ladies and gentlemen, please observe the fasten seat belt signs. Gentlemen please observe the fasten seat belt sign. Sir please observe the fasten seatbelt sign. Sir, yes you sir, about row 20, yes you, sit!! Thankyou