Building RESTful APIs is not enough anymore. My search for a better way brought me to Hypermedia APIs.
Don't version your API, version your API responses.
How to include sync functionality in your RESTful API without reinventing the wheel.
The book Running Lean will help you get from an idea to a sustainable business. It is not going to be easy, but this book just might make it easier on you.
Be sure to validate your in-app purchase receipts. The russian in-app purchase hack seems to be more widespread than thought.