I've got 10 advice which will help you in app promotion:
1)The app must be ready for submittal.
2)Leave space for feedback - encourage users to write reviews and rate the app.
3) In case of a paid application, it should have a free lite-version.
4) Apart from rating, include social network sharing into the application.
5) Social network pages are also good at promoting the app. Find relevant groups on Facebook, LinkedIn and Google+, use hashtags on Twitter.
6) Create an app trailer. Create a demo video. Place them on YouTube.
7) What also matters is maintaining the interest - that makes the app go on. You should regularly offer something new to users.
8) Build a supporting website for the application.
9) Advertising and promotion. There are a thousand and one good websites that can help you make the app known. Ad networks/PR websites, social bookmarking websites, app review websites, related forums, social media, thematic blogs, and more, and more.
10) At mutual will, you may have your application included in the developer's portfolio. That's a win-win - you both are additionally promoted.