Personally I've got a little "pax briefing" checklist on the back of my kneeboard which I use to brief them, usually once we're in the aircraft just before engine start.
I generally do my briefing AFTER the engine start, as I find 'less' of them jump out and run away! Also, on an unrelated note, when bringing passengers up who have never flown in light aircraft before I grab the checklist and advise them I'm just reading the instructions.
Starting to run out of people to go flying with though....