Just some thoughts (rough overview :D):
-I personally would wait till you have something “acceptable” to show → so which shows the core gameplay, style,… but everything in a decent way (showing too much is not always good)
-The “easiest”/best way to promote your game is to build up a facebook, twitter and youtube community → there you could connect with other indie game teams, let’s players,… to get a larger community.
-As you already mentioned, threads/posts in forums, indieDB,… will also help you to grow the community.
-You could also contact gaming networks (here in germany there is a tv show called “game one” → they also show indie games, but then those games have to be either super special or in a really good quality)
-But most important is that you have something really good or special to show → then the video/post will be spread over the entire internet, because people will like and share it.
e.g in case of this game they show something really special: https://www.youtube.com/watch?v=NbCtu_yt4kk
After you have a small community, you will also have to hold it → weekly posts, dev blogs, events,… just be present and let the people know that you are still working on the game