Create a header file like
extern NSString * const MyFirstConstant;
extern NSString * const MySecondConstant;
You can include this file in each file that uses the constants or in the pre-compiled header for the project.
You define these constants in a .m file like
NSString * const MyFirstConstant = @"FirstConstant";
NSString * const MySecondConstant = @"SecondConstant";
Constants.m should be added to your application/framework's target so that it is linked in to the final product.
The advantage of using string constants instead of #define constants is that you can test for equality using pointer comparison (stringInstance == MyFirstConstant) which is much faster than string comparison ([stringInstance isEqualToString:MyFirstConstant]) (and easier to read, IMO).