I think if your motives are honest and you can articulate them to the sitter (many people in my photos are not models) it will put people at ease. Most people I’ve worked with do not do it primarily for money, but for the ability to express themselves. They are genuinely interested in being a part of the process and engaging them goes a long way in building confidence.
I often ask models to help me to come up with the narrative for the specific picture or the entire shoot, kind of a short story in which they are actors. From my experience when a person is given a role to play she is less hesitant to put her own personality and her emotions into it, and she often totally forgets that she is naked in a strange place in front of a weird looking guy :-).