To be honest I'd say the best way to improve is to do
If you never actually run the game, you won't get better.
Even if it isn't perfect, a game can still be enjoyed. Just go for it.
The best game I ever ran on the forum is my first one, and I had just about zero planning for that.
The next few games had progressively more planning and become progressively worse.
I've heard similar experiences from Dexexe1234, who was a long-time MSPA and Bay12 Suggestion Game runner. Sometimes excessive planning, or laying complex rigid story bits out can paint you into a corner. A willingness to change or refine your idea over time is really important, as is letting your work grow organically.
On the topic of going into a project without extensive plans:
In "Take Joy", Jane Yolen (one of my favorite childhood authors) explains her creative process as starting with a seed. For example, it could be a scene; a Conan the Barbarian type takes tea with the Queen, which is a pretty ironic situation. For some people, depending on the medium, it might be a drawing or a song, a character or setting, or even a strong feeling. For Steven King's "Dark Tower" series, a poem he read in his Sophomore Year was the seed for his protagonist and his destination, and the rest of the Dark Western Science-Fantasy sorta setting extended from that.
Once an author has a seed, Jane Yolen suggested writers just allow their story to grow organically from it. Just think about how the thing came to happen, or what might happen because of it. How it fits in to a bigger picture, and what kind of Bigger Picture best fits that seed. As it grows, themes, plot hooks, challenges, antagonists, and character arcs might start to become apparent. If not, you can start to ponder those.
For my current Illustrated Suggestion Game, I started with a setting-based seed, a Scene, some Dice Mechanics, a Graphical Style. and some very long-term Story Beats I wanted to hit. We're only a few updates in, but the process of letting it grow has helped me create some little hooks, some near-future goals, and started to string out events pointing toward us a long-term twist and story shift. It's even changed some of the long-term Beats I had planned. For the most part, I'm leaving the content and scenes to be pretty flexible; I can shuffle around content to change the pacing, and add or react to new ideas on the fly. In general, I'm trying to treat the story as a living and growing thing, rather than as an extensive scaffold to drape some plot points and characters over (as I've done in the past).