Reading the code of frameworks / projects you like working with. It's especially interesting to look at older versions and see how it has progressed.
Design Patterns books tend to miss the practice with contrived examples. Real code focuses on problems people actually have.
Also, use other languages.
For instance abusing categories is something I learned from rails, when I learned of objc_setAssociatedObject I started adding properties via categories (which you aren't supposed to be able to do via a category).
Kent Beck’s Smalltalk Best Practice Patterns is a great book. In Smalltalk, but it works rather well for good OO languages, in general. Very easy to read. http://amzn.com/B00BBDLIME?tag=hboon-20
I haven't used it with Objective-C, but you should check out Smalltalk Best Practice Patterns. The two languages are close enough that it should work, Rubyists refer to it all the time.
