Use #defined values in Info.plist

In Xcode go into Build Settings set Preprocess Info.plist File to YES and Info.plist Preprocessor Prefix File to a .h file you've placed in your project with #define directives.

I usually use a Constants.h file that's included by PrefixHeader.pch also and define Preprocessor Macros for DEVELOPMENT, TESTING and APPSTORE that I copy over to Info.plist Preprocessor Definitions.

For example, to use different Facebook app credentials in testing and in production, #define the values in your header like:

#ifdef DEVELOPMENT

#define FACEBOOK_APP_ID             @"00000000"
#define FACEBOOK_APP_URL            @"fb00000000"
#define FACEBOOK_APP_NAME           @"App Display Name - Test"
#define FACEBOOK_APP_SECRET         @"abcdef00000000"

#else

#define FACEBOOK_APP_ID             @"00000001"
#define FACEBOOK_APP_URL            @"fb00000001"
#define FACEBOOK_APP_NAME           @"App Display Name"
#define FACEBOOK_APP_SECRET         @"abcdef00000001"

#endif

and set the keys<>values like:

<key>FacebookAppID</key>
<string>FACEBOOK_APP_ID</string>
<key>FacebookDisplayName</key>
<string>FACEBOOK_APP_NAME</string>
<key>CFBundleURLTypes</key>
<array>
    <dict>
        <key>CFBundleURLSchemes</key>
        <array>
            <string>FACEBOOK_APP_URL</string>
        </array>
    </dict>
</array>

Remember to Product > Clean when you change the defined values.