First keep Apple Human Interface Guidelines under your pillow.
Check also some of the most famous iOS apps and try to learn about their decisions.
But what is really important is to let many people outside of your team to try the application. Because your coworkers will become the masters of how to use the application even it has some flows.
I don't need any program, I use pencil and papers for sketching. This way I can sketch the UI very quickly without dropping tears when I need to get ride of it after spending a lot of time sketching it using a program.
But in case you need a program, check OmniGraffle, there are a lot of amazing stencils for prototyping iOS/Android and many other platforms interfaces.
Tapworthy is one of the best books about designing UI no matter what if it is an iOS or and Android app.
Tapworthy takes you from concept to polished interface design with plain-spoken principles and a rich collection of visual examples for designing exceptional interfaces for the iPhone and iPod Touch.