Build it and they will come....
Well, no. They won't. No matter how good it is, one app in a million will not be found and downloaded unless it is promoted.
App promotions is emerging as a complicated business. It involves elements of PR, advertising including in other apps, on websites and in targeted media, luck, reviews, fast-paced product renewal and extension, joint promotions that link products, even SEO-like activities on the vendor sites.
The most common promotion of paid apps is a free "light" version. This deals with a particular issue in app promotions, that the platform you sell on (eg, iTunes Sotre, GooglePlay) actually owns the customer, not you. So you can't offer a money back guarantee, which is one of the best marketing strategies for overcoming purchaser indecision. You can't even use selective pricing discounts. You'll just have to be more innovative than that.
Like all marketing, the real key is word of mouth. Your promotions should aim to facilitate and encourage people talking about your app.
There are some self-proclaimed experts in app marketing, you can find them on the web. We're still learning and the field is still evolving, so we're not making claims.
What we observe is that successful apps typically have owners who are highly involved in promotion, personally pushing for reviews, links, blog mentions, anything that gets people to look at their app.
Our advice is to plan your promotions from the beginning and start early on them.