Before you make any sort of promotional effort, you need to do the following:
1) Make sure you have a good rating. Get 10 friends to give you 5-stars. Nobody wants to download an unrated app (or even one with less than 5 ratings).
2) Add a video on the app store. It doesn't need to be pro, just something that gives a good indication of what your app does.
3) Make sure your screenshots are good and your description is accurate. The first sentence is the most important for describing what the app does.
After it's on the store (free version):
1) Post in forums
2) Submit it to review websites
3) Write up a press release (catchy headline, what your app does, why is it better - have a good angle) and send it to every news / tech / related website you can find. Who knows, 1 or 2 websites out of the 100s might write something about you, which can get you 100s of downloads. It's worth the hours it takes to do this. Also, use some Mail Merge software to make it personal (as simple as "Hi [name]").
4) Find influential twitter users that you think would use your app. Send them a message - dont harass them though. Be sincere. If they like it and they haven't tweeted about it, ask them to.
That's what I do. I'm keen to hear what others are doing.