I definitely agree with the advice to Go Narrow to Get Big, since that is what I do and it has worked great.

I provide accounting consulting and training to a specific group of businesses that are Govt contractors, meaning their customer is the U.S. Federal Government or they are a subcontractor to a prime contractor whose customer is the Govt.  This is the industry I worked in for 25 years before starting my own business,

so it is my specific area of expertise and so is where I can provide the greatest advantages to my clients.  

This niche area of accounting is not taught in general accounting classes and until recently this knowledge was only available to those you worked in the industry and knew someone who could answer their queries. Now there are training courses all over and I have thousands of competitors, but has not been a problem as there are not too many people who have over 30 years of expertise that are not retired yet.  So is all good.