I became a semi-active user after seeing an interesting question on Hot Network Questions, for which I had an answer to offer. The 30-ish upvotes it got over the next couple of days were pretty exciting (to put that into perspective, I came from SharePoint.SE, which has the lowest vote-to-post ratio on the network, where I had been as active as I could possibly be for several months, but still hadn't broken 200 rep there), and encouraged me to continue participating here.
So one way to convert visitors into participants is to ask questions that visitors would want to answer. I'm not exactly sure how to formulate that, but equally important is to convert one-time participants into repeat participants, for which I do have a suggestion:
- Keep an eye out for posts from new users
- When you see good ones, upvote generously
- On poor ones, hold off on giving them downvotes until after you've first made efforts to improve the post through comments/edits and given the new user ample opportunity to apply those suggestions
- Generally be welcoming and encouraging.
From my perspective, all of this is already happening here, so I'm really more or less just saying, "Keep it up!"
Also, with this site being less technical than most of the network, I think it may be easier here than elsewhere to convert a question for a very specific situation into a more general form (e.g. Explainer badge), which can increase the applicability of that question for visitors; this might help with conversion, too.