For me, it's Social Networking. ((Which being active on the forums, assisting others, giving feedback, etc etc etc are all a form of)) Maybe I overlooked it (only skimmed posts), but I don't think I saw anyone mention those big social networking sites like Twitter, Tumblr, or Facebook or whatever. I know a lot of people sort of roll their eyes at the idea of social networking (even if people are unintentionally participating in it on a daily basis), but when it comes to advertising your game, attracting new people, and spreading awareness that your game exists, getting involved on these pre-existing social networking sites is an invaluable tool. That's the way the world is moving forward right now and being noisy and noticeable about what you want to attract people towards on these platforms is huge. If it wasn't, big companies wouldn't have all gravitated towards it.
Years ago, before these sites were big, I had one tiny lecture centered around the concept of social networking and how it is the one best ways to be seen, known, and succeed in any sort of business or industry. It terrified me, because she was talking about going to community events, and such not, meeting people and up-talking yourself and your "product." I'm super shy and the idea that that was the new key for success was just plain scary. I really wish I had taken more away from that class, because I see now just how right she was and how important this is for being noticed with how the world is now.
I'm glad that this trend ended up being able to be conducted through a keyboard and the internet xD that makes it much easier for me to swallow and attempt. ( Though I really would love to go to some sort of Indie Game Dev Jam thing sometime ;o; )
Just like the forums though, it's a push and pull thing though. You have to give if you want to get.
tl;dr: Don't ever underestimate the power of Social Networking Sites.