Hear hear BlueLine!
If they don't enjoy it, there is no incentive for them to come back at all. We are surely here to sell flying - that's what I (try to) do. Even if it's not with our organisation, SELL FLYING AS AN IDEA.
"What we do in a trial flight is the first half of the first lesson, and it's basically a chance for you to (insert appropriate phrase here eg. throw the aeroplane around/have a bit of fun/see if you like it etc. - depends on the potential student's nature, which is gleaned in the first ten seconds of talking to them on their own).
You can do as much as you like, I'll do all the complex bits like the checks before takeoff, you can do everything else from taxi out, takeoff, fly around, not crash, even land it if you like" (and most of them can).
Then I tailor the rest of the briefing to suit them on the way out to the aeroplane. Preflight brief depends on whether I've already preflighted or not, their physics/maths background or area of interest, age and what they want from the flight. Are they interested in learning? If so I'll make it a bit more formal, and actually fly the first half of EoC. If not, then the next question is "Where do you live?" and the next one is "how adventurous are you feeling?" - possible chandelles (done very smoothly of course). ALWAYS SAY "you've got the nice touch you'll need" unless they try to snap roll you over the threshold, then it's "well, at least you're not intimidated by the aeroplane!" There's rarely nothing to complement them on.
I remember reading a story in Flying last year about airport kids - the ones who sit on the fence on final and watch the aeroplanes go overhead all day. We never know when we are going to get one walk in and say, "I've got enough desire to learn to fly, it's the only thing there is for me" so when they do we have a duty to the people who taught us to fly to make their work worthwhile. Maybe I'm a hopeless romantic, but I try to touch them spiritually. I did once today, and even though he's not quite 16 yet he's hooked. Well, and truly. He's coming back next week.
Safe flying
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Confident, cocky, lazy, dead.