Writing a persuasive message

When I write I like it to be short and sweet. I always feel that the longer I write the less I prove my point, especially if I don't have 100% familiarity with the subject I'm writing about. The persuasive writing assignment was difficult for me to write because I had to write more to persuade the reader to understand my point, and I felt as if I was just babbling by the time I got to the end.