Have done a few in my time, from my experience of what’s annoyed me.
If it was like one I did in the Surrey Hills, where the LBS leads a ride with you (and others) on their trial bike (actually mine was organised by the LBS but the riding was ‘contracted out’ to another local training / guided rides company but it amounts to the same). As above, don’t be a dick, moaning at the choice of trails / at people that want to stop and adjust shock settings, etc. People are deciding whether to drop a large sum of money on a bike and want to know if it suits them. If you wanted to just show off/just wanted a free guided ride, don’t waste the shop’s time on it.
And if it’s the type where they have multiple demo bikes and people for a day, and you turn up, leave some ID and a CC deposit or something and then take a bike out for an hour. Don’t take it out for 90 minutes instead because you liked it so much. Someone else is waiting for it and now I only get 30 mins on it before my next appt with another one. If everyone does that it royally **** up the whole process. Dick.