Disclaimer: Until I'm published, take any writing advice I give with healthy skepticism.
This morning I started to think about how I arrived here, days away from finishing a novel. Of the many things along the way that helped me, one stood out:
I told my ten-year-old cousin about it.
Aspiring writers, if you want to make sure you've got every corner of your world figured out, every facet of your plot bulletproof, tell a kid about it. You'll get an endless stream of questions, some so obvious that you would have never thought to ask them yourself.
Not only that, but every time you see them after that they'll ask for updates, or attack you with a new line of questioning they hadn't thought of before.
Now that I think about it, this strategy would work well if planning a jewel heist, too. I'll have to remember that.