Try to do these things
++ Send out a well-written press release to the media announcing your new app and its special features no one can do without.
++ Use your Twitter, Facebook, Google+ and LinkedIn profiles to promote the app several times in a week.
++ Create a product video
++ Pitch to app review websites
++ Identify bloggers who write on niches that relate to your app.
++ Apply for awards, If you win, you also get a ton of press.
++ Gather potential customer emails through Facebook, Twitter or your website. An email list of people who have opted in is a powerful marketing tool.
++ Promote your blog posts or content having to do with your app's niche on Dubbler -- a 60-second audio social network.
++ you can encourage people to tweet and share content on their social networks promoting your app. A random person every week can win a promo code to download your app for free if it is a paid one.
++ Be obsessive about resolving negative reviews.
++ Look for pages with 100k+ likes and reach out to their admins. Make sure these pages have something to do with the focus of your app. Then give them a compelling reason to mention it.
++ Provide a catchy one-liner about your app along with the link to download it in your email signature.
++ This will make it obvious to users to share with others. Path app allows users to share photographs on social networks and Draw Something makes sure you rope in a friend or more.
++ If you have a mobile website that gets a decent amount of traffic, it’s time to use that to your app’s advantage.
++ Feature the App in an Official Blog
++ Consider Alterative App Stores
++ Join Developers & Entrepreneurs Groups on Social Media
++ Make a paid app free for a short period (We ran a free promo with Wake Alarm last week: Wake was featured on Apps Gone Free and ended up climbing to #1 in the Utilities category and made it to the Top 20 Free overall chart. By the end of the two-day promo, Wake had close to 300k new users.)
++ Promote your app on your mobile website
++ Feature your mobile app on your website.
++ Get your app into the appropriate iOS and Android app stores.
++ Include a link to your mobile app in your customer service emails.
++ Discuss your mobile app in the appropriate forums.
++ Incorporate a link to your mobile app in your social media profile.
++ Promote your mobile app on social media.
++ Create a blog post explaining the utility of your app.
++ Post QR codes in your store, on your product and other offline media.