I think it was Toby Speed who put together an excellent briefing. Is she still around the red board? Haven't seen her here in ages.
I do all of the required stuff (seat belts, doors, etc.) and throw some humor in to help them relax a bit (This is a no-smoking flight, don't tamper with the lavatory smoke detector, etc...).
I also tell them what I'm doing, carefully choosing words to make sure they're not nervous. I think that's a big problem with some pilots taking newbies up, is they'll say things that are well-intentioned and designed to reassure them but have the exact opposite effect...
For example, I took a friend of mine (his 2nd time) and his daughter (her first). Friend had been with me on the flight that had by far the worst turbulence I've ever encountered, and he was a bit nervous. He turned around and explained to his daughter, "There might be some bumps when we get up in the air, but they're really not dangerous" or something like that. I clicked the Crew button in and said "Dude... Don't even say dangerous at all, even with 'not' in front of it. I know you're nervous, but you're making her more nervous!"
So, for example: Don't say "Well, the most likely time for the engine to fail is right after takeoff, so we're gonna run the engine up here and do some checks..." Instead, say "Okay, we're going to do a few things here before takeoff to ensure that everything is running normally."
Don't say "We did lots of engine failure training, it's no big deal." Instead, if they ask, tell them "Well, in the very rare case of an engine failure, the airplane will simply turn into a glider. I've practiced this and many other emergency scenarios to attempt to ensure a safe outcome no matter what happens."
Lots of other potential examples. Choose your words carefully.
One other thing I add into my briefing: "We're not stuck like we would be on an airliner - If at any time you're not enjoying yourself, just let me know and we'll come straight back to the airport." Giving them this sense of control helps. I also try to keep the first flight short enough so they never have a chance to get nervous. Generally, first flights consist of a quick city tour - Capitol, Monona Terrace, campus and stadium, their house, and back to the field. Leave 'em wanting more rather than getting over the high and getting sick/bored/whatever.