Here's a great example of how important following-up is:
I was representing the authors of Looneyspoons, a self-published low-fat cookbook. I kept following-up with People magazine, highlighting that the book sold 600k copies and had a great human interest story in the form of the authors' lives. Finally, we landed a spot - a front and back spread with eight photos! This is almost unheard of for this magazine's human interest spotlights. The book went on to sell 1.2 million copies.