Disclaimer: I do it for the love of what I’m doing and the learning and have never earned hardly a dime from my performances.
That said, I’ve met a couple folks who are really good at keeping pans hot in many fires at once - doing jingles, soundtracks, lessons, karaoke backgrounds, stock music libraries, sample and patch libraries, local and international performances, how-to books, recording gigs as engineer or producer, studio musician work … GADZOOKS, how they keep it all together and organized is just amazing! (But then again, they have a support network, and often a spouse handles much of the business stuff, and their “Roledex” is decked out with people they keep in touch with! I’m told one must dedicate four hours a DAY to following up on contacts and self marketing, the other four spent on nothing but music.)(Or art - I have friends similarly organized who make a living as painters and sculptors.)
Contracts are one secret - never, if you can avoid it, give complete control of a creation away. (Not traditionally possible in group-works like film, but possible in many other areas.) Sculptors were the first to pick up on keeping the rights to photograph their work as part of the sales contract in the late eighties, if I recall correctly. Many others in different creative fields have followed that inspiration since.
It is not my style at all - but I do know it is possible from seeing it done right before my own eyes!