If you don't know who Helene Parsons is, then you for sure know these places where her clients' work appears:
Dennis the Menace
The New Yorker
Harvard Business Review
Wall Street Journal
Better Homes and Gardens
First for Women
Chicken Soup for the Soul
Helene Parsons is a working gag writer for syndicated and magazine cartoonists -- as well as comedians and broadcasters. And she has started her own blog, Cartoon Gagwriting: My Experience.
I don't use gag writers myself, but some cartoonists do. By outsourcing the writing, it allows them to concentrate on drawing and producing material.
From reading books about joke or gag writing, I learned that they almost always had a set up and a punch. The set up is the straight, or factual part. The punch is the unexpected part and should come at the end. I also learned that certain words were funnier, specifically, words that had the "k" sound in them. Also, that there was something called a series of three, where the first two items were the set up and the third item was the punch at the end. After looking at thousands of cartoons in publications such as The New Yorker, Barron's and Reader's Digest, I could see that fewer words had a greater punch. I became familiar with the situations these cartoons focused on. For example, cartoons in Barron's mostly focus on the business world. I felt I could write business gags because after working in offices for many years I had opinions about things. I could see the humor in the relationships between bosses and workers, workers and co-workers, salary issues, lazy workers, lazy bosses. You get the picture. I also realized that I liked to write about restaurants (snooty waiters, demanding customers, lousy food) and relationships (husband and wife gags, mostly).
Go read Helene Parsons' Cartoon Gagwriting blog.