What are the major differences between how a behaviorist and how a humanist would motivate someone?

Well, a humanist (one prescribing to the theory that people are fundamentally good) relies on an individuals sense of propriety to guide them into appropriate behavior. So a humanist will appeal to a person's inner calling to "do good" for motivation.

A behaviorist will set up a system of payoffs in order to incentivize one into changing their behavior.