<configuration>
  <appSettings>
    <add key="LoginType" value="DAILY" />
    <add key="MergeAudio" value="TRUE" />
....
  </appSettings>
</configuration>

Then you can define, for example in a Utility class, the following method: it tries to read the key from the IsolatedStorage;
if it failes, it loads, from the configuration file, the possible default value defined by the application and then it save it in the IsolatedStorage for a quicker further use. In fact, next time that function will be called, it will find that key in the IsolatedStorage and the data will be available in less time!

        public static string GetAppSettings(string key) {
            //return (GetAppConfigString("App.config", key));
            //return null;
            String value = String.Empty;
            if (IsolatedStorageSettings.ApplicationSettings.Contains(key)) {
                value = IsolatedStorageSettings.ApplicationSettings[key].ToString();
            } else {
                // se non c'è nell'isolated storage: cerca di caricare, con GetAppConfigString, il valore di default (se esiste) da app.config, per poi salvarlo nell'Isolated Storage
                value = GetAppConfigString("app.config", key);
                if (!value.Equals(String.Empty)) {
                    SetAppSettings(key, GetAppConfigString("app.config", key));
                }
            }
            return value;
        }

Finally here it is the utility function to recover the default value of a key from the configuration file:

 private static XElement _xmlConfig;
 private static Object _xmlConfigSyncLock = new Object();

        public static string GetAppConfigString(string configName, string key) {
            if (_xmlConfig == null) {
                lock (_xmlConfigSyncLock) {
                    _xmlConfig = XElement.Load(configName);
                }
            }
            XElement element = _xmlConfig.Descendants("appSettings").Descendants("add").FirstOrDefault((el) => el.FirstAttribute.Value == key);
            if (element != null) {
                XAttribute xAttribute = element.Attribute("value");
                if (xAttribute != null)
                    return xAttribute.Value;
            }
            return String.Empty;
        }

When you need to get the key value from your code, you can simply call

 #region App.config keys
        public const String LoginType = "LoginType";
        ...
  #endregion App.config keys

string loginTypeString = Utility.GetAppSettings(Constants.LoginType);

and then when the user change that default value, you can update its value in the IsolatedStorage:

Utility.SetAppSettings(Constants.LoginType, LoginType.DAILY.ToString());

where, always in the Utility class:

 public static string SetAppSettings(string key, string value) {
   IsolatedStorageSettings.ApplicationSettings[key] = value;
   IsolatedStorageSettings.ApplicationSettings.Save();
   return value;
 }

Note that data written to the IsolatedStorageSettings object hasn’t actually gotten committed to the isolated storage area and it is saved only when the application that uses the class is closed. This can occur when the user closes the Web browser, refreshes a page, or browses away from the page. If you want your application to write to isolated storage immediately, you have to call the Save method in application code.