I'm already in the support program, and here's what I've done so far;
I post comic previews and early info on stuff I'm planning as locked posts for my supporters, and my plan is to give supporters early access to comic updates if/when the update system gets a bit more advanced.
Besides that, the milestone rewards include private livestreams, signed sketches, character designs, and comic cameos.
So far it's going well, and while I think that people support me mainly because they want to help me, the rewards might have incited some of them to donate more than they would have otherwise.