There isn't specific training on how to relate to clients per se. Most training is experiential, either through role playing with other students or during internship work. Most knowledge about human relations is learned through actual interactions and how successful and unsuccessful they are. Therapists are notoriously over confident in how good they are with people (according to research that I can't site off the top of my head, but I swear exists). To counter this, sessions during internships are often taped (with client approval) and critiqued by a supervisor. However, since people are so different in their opinions on the right and wrong way to interact with each other, different supervisors can have very different reactions to one session. I think this is why (as its been said before) therapists vary so much in how they interact with clients.