I make my living as a certain type of freelancer and I find myself talking to customers quite extensively and it can indeed be a bit draining. But I try to always be polite and honest.
I never promise things I'm not sure I can deliver and I sometimes deliberately set the customer's expectations lower than what I think I can actually achieve, so they often feel like I have overdelivered by a wide margin.
I notice that one of the things that inspire a lot of confidence and that has helped me close many deals is the fact that I'm very open about my limitations, about risks and caveats connected to using my services and about my own expectations and the role I expect the customer to play in the process.
Great observations... Illustrative of another old saw: "Honesty is the best policy." ;)
Thanks for sharing your insights, Dave!