One of the best ways, in my opinion, is simply sharing your knowledge and feedback with those starting out or are in a little bit of a rut. Stick with what you specialize as that will be the most valuable. I usually share my two years of social marketing experience with people who are lost in how to approach something like that that's outside their comfort zone. I've even responded to posts on /r/gamedev where people share their page or website and ask how to start marketing, and I'll give them feedback and steps to start taking toward early goals. If you put yourself out there and browse through forums like these, you'll find no end to devs looking for help/feedback!
I've also seen on the indie dev group on Facebook some people will donate time helping some early developers starting out and sit down with them to go over their code/concept art/whatever, so that could be something you can try