One of the best pieces of advice from check out Steve Marmon's (@marmon) Stanford lecture on How to Build an iPhone App that Doesn't Suck! (In 10 Easy Steps) is to spend time prototyping your app before you begin coding. Steve suggests using OmniGraffle, which I love or going the old fashioned route with pen and paper.
Check out Jason Robb's post on UX Booth's post on Tools for Sketching User Experiences for some great thoughts on the virtues of analog tools for developing UIs:
"Our brains are the ultimate tool for solving problems. But it’s hard to share ideas when they’re locked away in our heads. Sketching frees our ideas from our mind allowing others to see them. A visual representation of an idea has the capacity to be shared, collaborated upon, and improved. Whether I’m trying to show the hierarchy of an information architecture, or document the nuanced interactions of a web site, sketching has proved itself to be my most valued tool for giving form to my ideas."