A new book, "Yes!", has just published some fascinating information on how to get a "yes" out of people. For starters, people are more easily convinced when they are under the influence of caffeine. Another tidbit: a restaurant manager went from 30% of no-show reservation holders not calling to 10%. How? By changing the phrasing from "please call in advance if you can't make it" to "will you please call in advance if you can't make it?"
A trick we use in mock trial is to get the person saying "yes" to a series of question, which often leads to a slip-up "yes" later down the line (this tip is also mentioned in How To Win Friends and Influence People). This book could have important information for future PR efforts.