What I do is to post in-development screenshots and GIFs that show what I'm working on, and how it's going. It works well, but only if you have something to visually share, of course; if it's just something like code, or a console window that shows what a generated map is based off of or something like that, people won't be as interested, understandably.
A lot of promotion is just getting people hyped and eager to play, and getting their feedback and implementing it into your game as you can. Also, though, people won't get as interested if you don't really communicate with them; if you just come off as kind of a robot, then you probably won't make too much progress in making acquaintances. I made a little video showing how people can effectively promote their game and be good social media users, actually. Check it out, if you feel like it.