I\'m using iOS 5.1 I use this peace of code
[pathURL setResourceValue:[NSNumber numberWithBool:YES]
forKey:NSURLIsExcludedFromBackupKey
It's worth to note that you can't just set the flag on NSHomeDirectory(). It has to be internal to that path.
Finally I found the solution by myself The clue is to apply NSURLIsExcludedFromBackupKey to root folder not to every file you want to exclude from backup
so at very beginning you should call this with (for example) "Library/Application Support" folder