A significant amount of my downloads come from my signature / avatar on MacRumors. Every time I post in any thread, it's like I've permanently plugged my ad into the thread. If I make a particularly likable post on a front page story, I can get propelled to the top of the top comments list, which leads to a fairly prominent display to the generally iOS/Mac owning community of MacRumors. I can also change my signature, thus changing my personal ad all over MacRumors, so, for example, I can easily promote a weekend sale on my app (don't count on it, sales have been doing well, it's not dropping back to $1 for awhile... I might just increase the price to $3 though after my next update.)
Other promotional things I've tried...
YouTube. Total flop. Unless you have an amazing video that people are going to share with each other, don't bother. As an example of this kind of ad done right, here's an ad for Songify; you may have seen it before if a friend forwarded it to you:
Have a FaceBook page. It has worked out okay for me. I've only got 12 likes and a few posts/messages from other people, but each time they post or like it, my app gets plugged into the news feed of their friends.
Also, you could give out some free codes over here to try and generate some interest:
https://forums.macrumors.com/forums/136/