A Purser in my company uses to say, after the safety card demo briefing ,"please read this card carefully as a written exam will be performed during the flight".
For the life jacket demo: "a beautiful yellow life jacket is located under your seat. Place the life jacket over your head and pull the straps down. Cross them around your own lovely body and tie them forward etc etc".
Also, before night take-off and landings: "the cabin lights will be dimmed.Please don't take advantage of the situation".
Each time I flew with him passengers were delighted as this Purser relaxes passengers in a very professional way.