I tend to keep the briefing fairly.... brief!
I tell the passenger not to touch any controls, and to tell me if they touch something inadvertantly. Most important is keep feet well away from pedals.
I explain that in the unlikely event of an emergency landing, it may be necessary to unlatch the door just prior to touchdown to prevent the door being jammed if the airframe bends (but this is dependent on a/c type - it's OK in C150/152/172 or PA28, I would probably NOT do this with a bubble canopy that's hinged at the back, like a Katana!).
Advise the passenger to let you know if they feel ill, and as mentioned in a previous thread by Aerbabe (I think) keep an eye on them just in case they go pale or quiet, or start to sweat (sure sign of airsickness or terror, or both).
Tell the passenger how to adjust their seat but ensure that it locks in place - no good having the pax sliding forward into the control column as you start your descent! Show them how to fasten and undo the seatbelt. Tell them NEVER to touch the prop. and to always walk away from the aircraft rearwards (unless a pusher prop aircraft of course!) when getting out, and to keep clear of other 'planes/taxiways etc.
I think that's basically what I did on Wednesday evening when I took up my boss for the first time. The bit I forgot until back on the ground was to tell him my demands for a pay rise and my threat to operate the ficticious ejector seat if he didn't agree.
