That's an excellent question.
Primarily, this would be accomplished by erecting barriers to practice, such as licensing and regulation. That's a two-edeged sword, because it gets government into the profession, and creates a situation where commissions are established that begin to dictate so many requirements that you can find yourself being limited in what YOU can do.
In Pennsylvania, we had a wild west environment for many years in real estate. When it came time to suggest regulations, it was the large brokerages that did so. They essentially prevented newcomers from opening their own brokerages without going through many hoops. Then there is the consumer requirements flowing down from HUD. Some of it is good, but there is no requirement to use a professional at all. This created room for web developers to sell real estate advice without being in the practice of real estate, competing with agents on a cost basis that is impossible. "Bubba" on steroids.
Perhaps a better approach is to form a regional/county professional association and pony up money for advertising that says, "make sure your air conditioning contractor is a member of blah-blah, ensuring quality and value for your air conditioning needs."