My Global.h can look something like this:
Same when I need an images, simply typing kImage... and code-completion gives my a list of all my defined image names. No more going back to check what the filename was or looking for bugs that are caused by wrong spelling in the key string (maybe I'm the only one who get this kind of bug *_* )
If Xcode doesn't show the auto-completion list, try hitting ESC. If still no suggestion comes up, the needed file is not included or you're trying write something that doesn't belong there. On rare occasions the auto-completion systems fails to work properly but building the project should solve that.
If you have any good tip on how to make the most of Xcode, please let me know!