Pretty much like to SEO any web content, the first step to SEO your APP no matter its a iPhone, iPad, Android App or Blackberry App, always starts with proper keyword research.
Ask yourself the following questions before you will come up with the primary keyword term:
1. What’s the best category of your app?
2. What OS platform is your app good for?
3. What problem does your app solve?
4. What’s the uniqueness of your app? Comparing to similar ones in your category.
5. What is the name of any related app which ranked on the top?
When you have your primary keyword term to your best knowledge, put them in Google search to find its competition level, which is how many search results do you get for that primary term? Add modifiers to that primary keyword till you will find some variation keyword phrase whose phrase match search result is less than 100k in Google. At the same time, you’d better find out the actual search volume for that variation keyword phrase. If it comes up with decent amount of search numbers, you want to use that one for your app marketing.
I suggest that you should come up with 3 to 5 keyword phrases to put in your app title and description. And when you promote your app, use those keyword phrases as the anchor text links to your app download url or to your app review links.