Previously, in What does building a mobile application involve? - Part 2, we talked about coming up with that perfect idea for an app. Now that we have that idea, we'll talk about what to do in the app design phase.
Before jumping right into coding or hiring a developer, it's always good to have some sort of draft or storyboard to start from. This can be as simple as a couple sketches on a napkin of what you want the app to look like to as complex as animated Keynote/Powerpoint presentations as demonstrated by the prototype templates at Keynotopia. Personally, I'm a HUGE fan of the iPhone app Dapp which enables you to actually create a working app (with limited abilities, of course), preview how the app will function and even export the code for use in a full iOS project. I find this app wonderful for quickly putting together app navigations and table views.
Anyhow, whatever prototyping/wire-framing/storyboarding method you decide to use, use it. This will be indispensable to you (or your developer) during your initial stages of app creation. Just remember, it does not have to be exactly like the finished product will look. In fact, you may go through so many design iterations that your initial draft looks nothing at all like the final result. However, anything is better than nothing when you're starting out.
Another thing to consider when designing your app is its usability and intuitiveness. One of the reasons the iPhone and iPad became so popular was the simple sophistication of the apps developers created for them. Apple created a set of guidelines and behaviors it expected app developers to follow and by following these rules, developers created apps which need little to no instruction to use (for the most part). This is due to the fact that users became accustomed to the way apps on the devices should perform/behave; they quickly learned what a back button would do or that a table could be scrolled up and down and its rows clicked on to perform another action. When designing your app, you should keep this in mind. You may think you have the greatest idea since the TV Guide Channel for a user interface but will that great new interface be usable by your users who're so used to certain functionality paradigms? Food for thought. So, rather than talk in further vague terms, why not send you to the master itself? Freely available on it's developer website, Apple has posted their iOS Human Interface Guidelines which they expect all submitted iOS apps to adhere to. Even if you're not into the iOS thing and are planning on creating a web app or creating an app for another mobile platform, the principles laid out within Apple's guide are universal. This is a GREAT starting point for your app's design.