I usually promote my Patreon page as an add-on to posts elsewhere. For example:
I recently posted a rough sketch of a new painting, and added on that patrons would be able to watch the progress as I worked, with a link to Patreon.
I posted again at the halfway point, with the same link, saying patrons were getting several progress photos a day, with commentary, and were able to ask questions and receive long, thoughtful answers.
When the painting was finished, I posted a direct link to the speed-through video on Patreon, showing it being painted start to finish. I mentioned that patrons had access to a dozen progress photos.
Something I was doing must have clicked, because after months of no new patrons, two new people joined up while I was working on my latest painting.
I use Buffer to post to Facebook, Twitter, G+ and Instagram, so I can write up the basic text and upload one photo, and then tweak and add hashtags for each service. Since IG doesn’t allow links in posts, I have a Patreon link in my profile.
I also host art exchanges, and at the bottom of each email I send to the participants, I mention that patron pledges help offset the time it takes me to organize collaborative projects, so if they’d like to see more of them, consider joining the tribe.
I try to be creative, and give people excuses to “join us”, telling them what they’re missing out on, rather than just asking them to become a patron.