Many other instructors have been prosperous in their specialty, too. Mary Bryant of Eclipse International, Inc., in St. Petersburg, Florida, is good example of how an independent CFI can be successful. Mary offers specialized training in the Piper Malibu, Piper Mirage, and the turboprop version of these airplanes. She provides the transition training that's often required by insurance companies as well as annual refresher and recurrent training. If you want to learn the specifics about these airplanes, you won't be disappointed when you fly with Mary and instructors of her caliber (please forgive me for not mentioning all the other fantastic CFIs who specialize in different airplanes or aspects of training).
One reason that instructors like Gene and Mary are successful is because they've become specialists in a particular aspect of flight training or in a few specific airplanes. You might consider doing something similar. There are many areas in which you can specialize - CFI preparation, aerobatics, or taildragger transition, just to name a few.
So make it a point to master one or two areas of aviation training. You'll develop a reputation in your area of expertise that will spread by word of mouth. While this isn't the fastest nor the easiest form of advertising to acquire, it's one that works for you 24 hours a day, seven days a week.