There is perhaps no more powerful way of influencing others than to tell stories. What I mean is that no matter how you're attempting to influence, use the example of others around you to make a point.
For example, a software client was having trouble with someone on their sales force who was producing but was a disruptive influence and was, in my opinion, hurting the efforts of the sales group. And so I introduced a story of my experience with a similar person I worked with in a sales organization at AT&T. I described the short term gains but long-term pain associated with the person. This helped fuel the dialogue around the rep's future with the company.
In selling situations, I have typically used another person as an example of how they came to decide on the product after working through some doubts and the example has translated to the person I was selling to. Through the introduction of a similar example, people are less likely to be defensive and actually welcome communication that will help them to better understand.
Wise people use stories to relate because they know people are comfortable with them. I suggest that you use this in marketing, sales, and other communications situations. A good example of this in the marketing sense is the use of case studies.
Do you use case studies? How have they worked?