1.Command-Option-Shift-K to clean out the build folder

　　XCode4.2    finder中找到   /Users/apple/Library/Developer/Xcode    （注：Library资源库是隐藏的文件夹）

　　里面有DerivedData和Snapshots两个文件夹，删除里面文件即可

　　Remove all its contents because there's a bug where Xcode will run an old version of your project that's in there 　　somewhere.

或者  File->project settings

2.(Xcode 4.2 will show you the Derived Data folder: choose Window > Organizer and switch to the Projects tab. Click the right-arrow to the right of the Derived Data folder name.)

In the simulator, choose iOS Simulator > Reset Content and Settings.