I'm gonna say it: the best way to significantly improve as a DM is to practice and ask for feedback. This is why I stopped running RPGA games when the old "rate your DM" practice stopped; I probably ran 300+ RPGA games, and getting 1500+ sheets of paper saying what you can improve makes you recognize what your weaknesses are. I still have a lot of weaknesses - the only difference is that I'm deeply aware of them.
It's not a bad idea to spend five minutes after every game asking yourself "What might I have done differently that would have made the game more fun?" Then do that next time and see how it works.