While lots of people are suggesting creating communities for your niche interests, I think it's even more important to to find niche communities that others have created and contribute to them. Obviously you can do both, but if you've got limited time to post it may be better to focus your efforts, and be the "first follower" rather than the leader.
I've been doing this for /m/Animemes and /m/anime_irl, just making one post per day in each. There hasn't been a ton of other activity yet, but the subscriber counts have been growing steadily, so we'll get there.
I've also been wanting to build up /m/Bitcoin in the same way, but I don't feel like I've got much to contribute right now, so I'm focusing on the anime communities.