In this talk, I covered why reusing and keeping your code DRY is import. Apple's provided mechanism for this on the iPhone is static libraries. I went through the anatomy of a static library (and its friends). The talk ended with a demo showing how to setup an iPhone application and then abstract out a class into a static library. I also showed how to setup direct dependencies and everything else needed to get going reusing your code.
This talk was based on a screencast I did awhile ago. You can get the test app code and test library on GitHub.
I talked a bit about the open source library that my company Tasteful Works is I am distributing, SSToolkit. You can also get its source code on GitHub.