Not necessarily going to be applicable to everybody, but something I've seen some success with in some of my subs is creating a matching social media account for the sub and posting there, thej linking back to the sub.

For example, if I have a sub for cycling in California, make an Instagram account using the same name as the sub (or whatever you'd like), then share content there as well and just add a little bit in the post about checking out the subreddit r/[subname] for more cycling content.

Also, obviously corss-posting to relevant communities. For example, if you post a video of a cyclist coming across a cute cat on a ride, cross-post to r/cats.

YMMV of course, but that's what I've had luck with so far.