The mobile landscape has never been more competitive. Know that you're delivering the right set of features with the right experience. Understand your target users, their preferences and habits. Get it right before you launch. Streamline the on-boarding process. Get a prototype in front of your users and capture meaningful feedback.
Design the entire experience end-to-end. Nail the high priority use cases. Address important edge cases. Wireframe everything in between.
Target the priority devices, but be responsive for every user's device.
Should you go with an app or will a mobile web site suffice? Understand the differences, costs and trade-offs.
Users don't need another app or website in their lives. They are already overburdened, bogged-down and "cognitively overloaded". You need to come in and sweep them off their feet.