Joint ownership
Before you create intellectual property (a book, a song, a patent, the words on a website, a design) with someone else, agree in writing about who owns what, who can exploit it, what happens to the earnings, who can control its destiny.
This is sometimes an uncomfortable conversation to have, but it's far worse to have it later, after the thing you've created has been shown to have value.
It's almost impossible to efficiently split a soup dumpling after it's been cooked…