Constants in Objective-C

I'm developing a Cocoa application, and I'm using constant NSStrings as ways to store key names for my preferences. I understand this is a good idea because it allows easy changing of keys if necessary. Plus, it's the whole 'separate your data from your logic' notion. Anyway, is there a good way to make these constants defined once for the whole application? I'm sure that there's an easy and intel

[tasanobu]

extern NSString* const

[komagata]

静的クラス変数の書き方

[tomute]

Objective-Cで定数を定義する方法