First of all, you need to have an attractive name. Ensure that the app is easily searchable, via Google or app store, by a casual user.
Begin by posting a demo version on popular sites, such as Facebook, Twitter or Youtube. Tell your friends to do the same for you and keep tabs on the responses you get.
Create a free version first, rather than going pro. For starters, it is necessary as user tend to look for those apps which will cost them nothing. If they find it attractive, and interesting, ultimately they will be more than willing to buy. Make sure that the pro version has some extra features, including a demonstration element, which makes it relatively user friendly.
Market your app by using all possible tactics. It is the most essential step. It does not simply include cost-effective measures, such as social networking sites, but you will need to create a budget for yourself and think out of the box. Promote by creating your own blog, where you can answer any queries, and advertise your product extensively by setting up a website. Use the 2D mobile barcode method where you can directly link with the iPhone App store.
Contact some known iPhone experts and ask him to give a review of your app. Exchange some ideas and use it to suit your need. In most cases, word of mouth remains the major marketing source, which helps you promote your app quickly and effectively.
Check the responses you get through your own website. It will help to create your own fan base and further encourage customers to promote your app by bringing in more users. The website will also help you analyze your target audience and cater to their specific needs.
Get to know the use of Smart App Banners on iOS 6, which allows you to promote your app when a mobile user accesses the Safari browser. The user is automatically directed to your website.