I’m curious to hear about the lessons people have learned along the way as GMs.
Tell us about something you used to do that you have since determined was wrong/dissatisfying/mistaken.
How did you come to this realization?
What steps did you take to improve?
How have things gone since?
The biggest mistakes that I've made in my GMing is letting terrible players into the games I run. Not just terrible players, but terrible people as well. Real bad seeds. You know the type I'm talking about?
Beyond that, one thing that I used to do wrong that I've fixed now is that I used to call PCs by their actual names. That was terrible and stupid and bad. So I've stopped doing that. Games are typically better if you just call PCs whatever the hell name you feel like in any given moment. Bonus points for calling one PC the name of another PC. Marked improvement on play.
More seriously:
* When running map & key crawl type games, the first thing I think about when I'm generating the crawl is spatial dimension, in particular multiple ingress and egress points for most every room. It is infinitely easier to work out the rest of the details after you've nailed down a functional scheme of multivariate ingress/egress and connectivity relationships within the architecture. The theming, stocking, etc should be worked out after that.
* When a game possesses a Twist/Success w/ Complication or Cost scheme in action resolution, developing a good shorthand (either cognitive or physical) that indexes the needs of the game in question.
*
Don't let perfect be the enemy of good. Obsessing over the perfect anything in gaming (whether it prep or in-situ exection) makes for either stifled play or unsatisfactory experience of play. You're going to have moments, sequences, sessions where you weren't at your best. Its ok. If you freak out about it or obsess over it, guess what? You're going to actually increase the frequency and magnitude of those less-than-stellar moments. You don't need to reduce your expectations for play. You need to reduce your unrealistic expectations for play.
Those are three ways I've improved my GMing execution or experience.