Before leaping into your latest app idea, it's a good idea to stop and consider what you are trying to allow the user to do, and figure out how they will actually do it. One of the best ways to do this is with the decision tree or navigation tree. These can be easily drawn on paper, or if you're collaborating with other people, index cards on a magnetic whiteboard (or even a pin-board) are very effective.
The idea is to not just draw out the possible screens in your app, but how the user will get to each one. Navigation diagrams don't just help to define what screens your application actually needs, but will help ensure that the user will never be "lost" in your application. If the navigation lines become too complex, then you need to simplify the navigation (possibly by adding or removing some of the screens). Overly complex navigation is often hidden in the use of an application, but when drawn on a diagram, the complex relationships between screens become obvious, and...