It's definitely not an exact science, you're right. But it's not about reading the person, at least not for me. I tune in to a person's spirit guides, who talk to me and show me things about the person and what they most need to know. I then convey what I'm told and shown. Sometimes I know in advance the sitter won't resonate with the information but I pass it on anyway because I feel honor bound to pass on whatever the guides give me. Most of the time what I tell them does resonate.
After the guides have had their say (and that sometiems take 15 minutes or longer) I then open it up for people to ask their own questions. But in the beginning I don't even let the sitter ask a question or tell me why they've called. I need to have no information when I start. That way when validating evidence comes through I know it's from the guides not my sitter.
If you haven't read this blog entry yet, now would be a good time. It contains some info relevant to this thread.
Psychics Are Like Doctors