One of the simplest bits of advice I can ever give someone who is looking to be a better GM is that you can only get better with practice. Much like painting and sports, practice hones a GM's skills and subconsciously teaches them how to adapt to real-life situations.
Book learning has its place of course, but often the best way to pick up confidence and further develop a personal style is through constant practice. Think of it as similar to being a pilot. Pilots require hours of flight time, as GMs, we need hours of running and preparation until we find what works best for us and our players.
I started off GMing for a college group that hung out in a local gaming store. We were a group of fans of the L5R CCG and a friend scored a copy of the first edition of the RPG. We quickly learned to play the game and took turns GMing, round-robin style. Eventually some of us learned that we enjoyed GMing more than playing, and others the other way around.
With a few hours of one-shots under my belt, I figured that I could aim my sights a little higher and run a campaign. It was the first one I'd ever run, and perhaps one of the most blatant rip-offs that I've ever made, stealing liberally from Blade of the Immortal, Rurouni Kenshin and Ninja Scroll in one action-adventure with improbable characters and terribly cliche dialogueand I loved it.
Of course I had all sorts of mistakes in the way I ran. My plot was meandering, villains were paper-thin cutouts that existed for fighting and I railroaded the hell out of my players, but everyone had to start from somewhere.
Sure my players loved the games, but I never really thought of settling into a pattern. I picked up Mage: the Ascension at the recommendation of a friend, and I learned that you could run games to focus on different themes and moods, and that set me on the path of always moving towards improving how I play.
I've had my fair share of successes and failures since then, but there's always room for improvement. In my case, improvement only happens the more I run, so I can't really stop and settle on my style as is. It keeps the hobby fresh and interesting to me, and I hope that other GMs work with that mindset as well.