How to declare and use NSString global constants

Possible Duplicate:
Constants in Objective C

I store some app settings in NSUserDefaults. NSStrings are used as keys. The problem is I need to access these settings throughout the app using those NSString keys. There is a chance that I mistype such string key when accessing in some part of the app.

Throughout the app, I have such statements

[[NSUserDefaults standardUserDefaults] setBool:YES forKey:@"ReminderSwitch"];

BOOL shouldRemind = [[NSUserDefaults standardUserDefaults] boolForKey:@"ReminderSwitch"];

How and where can I declare a global NSString constant which I can access throughout the app. I will then be able to use that constant without worrying about mistyping those string keys.