Not a huge income generator (around $19,000 total over the last 5 years), but the pose research, networking, booking sessions, actually doing the modeling, and reporting on / posting the artwork from my sessions on my social media is pretty much a "sole passion" for me.
To keep it interesting for me and my artists, I regularly scan the Instagram posters I follow, as well as other sources (like classical art sites), and save interesting ones I'd like to try.
Then I run a slideshow of the poses on my home TV. I'll take a moment to look as I pass by while doing chores, or I'll spend some dedicated time actually doing the poses for timed intervals.