I tend to brief that they are engaging in a hightly dangerous activity and are putting their life on the line from the moment they step onto the apron - that is of course why they need to don yelow jackets and why we will form a cordon around them as they make their way across the apron.
I then explain that they will feel sick, they will be thrown about in the aircraft from the moment we start the engine until after we stop the engine and they will instantly develop an insatiable desire to go to the loo - which I will explain we do not have. With luck this should make their desire even more insatiable. I will encourage them not to open the door to relieve themselves, even though I am about to thoroughly brief on opeing the door.
I will point out that they will hear all sorts of alarms going off, can expect to panic during every phase of the flight, must make sure they know how to exit the aircraft blindfolded and may only speak when told to unless of course they hold a radio telephone licence.
I will be at pains to give them a detailed explanation of the mechanical characteristics of aero engine and the physics of a wing and point out that its an aircraft in which we are sitting and not a plane, because if it were I would be a carpenter and not a pilot even though there is a hammer. Of course I will then proceed to instruct them in the use of the hammer.
Finally I will point out that I will shortly be handing out multiple choice questions on the afore brief. Anyone achieving less than 95% will be ask to sit in the corner of the hangar for the duration of the flight.
I think that about covers it, doesnt it?