Never give up!
Never give up on a prospect that you know is right for your station’s event.
Source: Brandeis C. Hall, RAB
If you have a sponsor that is difficult, ask him/her, "What can I do to get you involved?" Let them tell you what is appealing to them and how they can benefit from the participation.
You would be surprised how many times they will tell you exactly what will make them happy. Then all you have to do is deliver!