Good design is an iterative process.
The more you exercise your interface design concepts early on in the process (before you write any code), the better the end results will be.
It’s also important to optimize your design for your target audience and the target device. Great apps compensate for the user interaction concepts that will vary with the device’s unique characteristics. Optimizing your app is all about refining and iterating on these concepts so that the end result will delight your users.
Before you can begin to successfully build an app, you'll need a solid set of blueprints. You might start off with some rough sketches and then refine your ideas over time. With each turn you’ll discover more about how your user might interact with the app and new ideas that you could incorporate—all without spending any time, money, or resources on actual development. Consider drawing or sketching out your entire app flow, beginning to end, to get a complete feel of the user experience as well as the functionality your design will create.
iPad vs. iPhone
If you're planning to develop an app that runs on both iPhone and iPad, you need to adapt your design to each device. While most individual UI elements are available on all devices, the overall layout usually differs dramatically. For example, users tend to expect more high-fidelity artwork in iPad apps than they do in iPhone apps. Merely scaling up an iPhone app to fill the iPad screen is not recommended. Instead, you need to make your iPad app engage the user in ways that take full advantage of its larger screen and capabilities. The devices also support different UI elements. For example, popover controllers or split view controllers are unique to iPad.
“iPad definitely gave us more real estate, which we wanted to take advantage of. That was key for us. It was not let's just make everything three times as big, but let's really make sure we're properly using that space... If we're going to translate one from an iPhone to an iPad, we re‑think it.”
— James Blomberg, General Electric