As J. Patrick Hazel writes--
Unless the witness is a professional testifier, terror is likely the witness's dominant emotion. Ask a few questions to ease that feeling. Ask things that cannot be challenged by the opposing lawyer and the jury. "Where do you live?" "Where do you work?" "Are you married?" "Do you have any children?" "Where do your children go to school" are a few such questions. Even if the witness goofs in answering these questions, no one else will know.
Source: "Direct Examination," by J. Patrick Hazel, in The Litigation Manual: Trial.
Related post: "Another Book to Think About: The Litigation Manual: First Supplement."