Kill the clutter:
- Embrace the constraints of a small screen! Zero in on doing one or two functions in an amazingly focused way.
- Strip out features that are not absolutely essential to the functioning of the app. Less is more.
- Break the interface up into logical screens and don’t try to do too much on one screen. Though minimizing the number of taps required for a function is always optimal, sometimes functions are easier to understand if they are split up into multiple screens.
- Only display UI elements that are essential for a given screen to function.
- Padding and whitespace are your best friends.
Design for context. Though contexts vary, there are some general mobile design principles that are often ignored:
- Design big, bold, and minimal. Folks generally aren’t paying full attention to the interface and are often using it while doing something else. They don’t see the interface, they see a blur. Make the important elements pop.
- Beef up the font size. Don’t let the simulator deceive you; fonts look much smaller on the actual device. Be careful not to limit testing to retina-display devices like the iPhone 4. Smaller fonts look great on retina displays but absolutely fail on older devices.
- Do one thing really well and trash the other features. If you’re porting a desktop app or website to mobile, don’t just optimize for mobile; rethink the entire experience.
- Our fingers are very big and imprecise so make targets big.
- When designing forms, don’t forget the digital keyboard is going to take up about half the screen when the user taps on the first field.
From Jeremy Olson: 10 Surefire Ways to Screw Up Your iPhone App