I have recently decided to try and do this professionally, so I have literally been working to improve my skills by running more games for more people. However, because this is new and uncharted territory for me, there isn't an obvious plan or roadmap to follow for success. So I had to come up with my own ideas and give it a shot.
(I realize this is not the same thing and could lead to a whole different topic for discussion, but the points about improving your GM skills are highly relevant to this. To that end, I will not discuss business-related or professional advice here. Just focus on the GM stuff.)
I know that I am good and competent at what I do. My own personal style and unique personality help to distinguish my own brand, so to speak. But as soon as I decided to make a serious go at this, my first step was to figure out ways to prepare myself as a professional as opposed to someone who just ran more games for strangers, fun, and profit.
I have been running games on a message board for the last few years due to my work schedule, but that recently changed. So the first thing I did when I got more free time (and my wife's support, of course) was to put myself out there again for more face-to-face games with people I did not already know. I first considered 5e D&D because it gets more interest and there is always a demand, but then I decided to see if I could get interest in the game I truly wanted to run, Edge of the Empire. Sure enough, there was interest and I quickly found a dedicated group of players. I have been running the campaign for them every other Saturday since August, and we have become good friends.
I treat every session like an audition and a training exercise. I pay close attention to my performance and read the reactions of my players to provide clues to what works and what needs improvement. Since I began, I have continually focused and improved on my pacing, organization and prep, and my ability to focus less on the rules and books to capitalize on more play and fun during time at the table. Turns out the group is a lot of fun and the campaign is still going strong.
I have also run a couple one-shots to teach others who were curious about the system, and try to get them started with their own groups. I even did a GM demo recently to demonstrate the mechanics of the system and how to incorporate some of the fundamental principles of improvising scenes, collaborating more with the players to build a scene, and using theater of the mind more effectively by removing visual aids like maps and minis (and also when maps and tokens are actually useful).
Now that I have gotten my stride, so to speak, I am getting ready to ramp things up with more games and more groups. I want to be able to focus on several ongoing games and systems at a time to become more diversified and able to accommodate more demands. This may require more time management on my part, finding ways to simplify prep and being more prepared. I also need to be more knowledgeable with rule sets, or at least comfortable enough to adjudicate on the fly as needed.
In short, the first thing I asked myself when I chose this route was "Why would anyone want to pay me to run games for them?" And that has become my roadmap, not only for my future professional career, but also for my mission of self-improvement.
There is more I can share, but I have babbled long enough. I am interested to hear ideas from others.