Identify the problem you are trying to solve in the design. Is it to accomplish a task, to enjoy a free moment, to communicate or capture a moment? Have a clear-cut problem statement and direction for what you are designing.
Target your users. Identify who you are designing the app for, and understand who they are so you can provide an appropriate app for them. Interview representative users to get to know them better.
Keep context in mind. Don't try and overload an app with functionality that is not appropriate to the mobile context of computing "out and about." Focus on designing for that context and also…
Keep your design simple. Don't overcomplicate the design – you have a small screen and if people are using it "on the go" you need to provide big target areas and obvious affordances.
Finally, follow (or don't follow) the UI conventions of the mobile OS. If you want people to quickly learn your mobile app, follow the conventions of the device's operating system. If you have a dramatically different idea on how to design your app, follow that idea, but keep it internally consistent to support learnability and usability.