From time to time, during repeated testing and debugging, and before making a different sort of build (switching from Debug to Release, or running on a device instead of the Simulator), it is a good idea to clean your target. This means that existing builds will be removed and caches will be cleared, so that all code will be considered to be in need of compilation and the next build will build your app from scratch.
The first build of your app after you clean will take longer than usual. But it’s worth it, because cleaning removes the cruft, quite literally. For example, suppose you have been including a certain resource in your app, and you decide it is no longer needed. You can remove it from the Copy Bundle Resources build phase, but that doesn’t remove it from your built app. Only cleaning will do that, because it removes the built app completely.
To clean, choose Product → Clean. For more complete cleaning, hold Option to get Product → Clean Build Folder.
You should also from time to time remove all versions of your built app from the Simulator cache. Choose iOS Simulator → Reset Content and Settings. Alternatively, you can clean the cache by hand. To do so, first quit the Simulator if it’s running. Then find the cache in ~/Library/Application Support/iPhone Simulator, followed by the system version of the SDK (for example, there might be a folder called 4.3); within this, find the Applications folder, and move the contents of that folder to the trash.
In addition, Xcode ...