MonoGame - when/how to save data

Question

I am making a simple 2d game for WP in monogame/xna and am at a point where I want to save data. I have created a method to serialize the data (index, counter and percentage counter) to an isolated storage file in xml format. There is also a subsequent method to deserialize the file. My question is simply, what point is best to save the game, because at the moment I save when the a pause screen is loaded over the gameplay screen - However, this wont save the data if the player exits the app. I am certain I am saving in a silly place. Hence, I am wondering what the common practice is for saving? Any help/advice would be greatly appreciated.

Here is my save method:

    public void SaveData()
    {
        const string filename = "data.vision";
#if WINDOWS_PHONE
        IsolatedStorageFile dataFile = IsolatedStorageFile.GetUserStoreForApplication();
#else
        IsolatedStorageFile dataFile = IsolatedStorageFile.GetUserStoreForDomain();
#endif
        try
        {
            // Create an isolated storage stream and initialize it as null.
            IsolatedStorageFileStream isolatedFileStream = null;

            // Check the data file already exists.
            if (dataFile.FileExists(filename))
            {
                // Open the file using the established file stream.
                using (isolatedFileStream = dataFile.OpenFile(filename, FileMode.Open, FileAccess.ReadWrite))
                {
                    // Read the data from the file.
                    XmlSerializer serializer = new XmlSerializer(typeof(Data));
                    // Store the deserialized data object.
                    Data savedData = (Data)serializer.Deserialize(isolatedFileStream);

                    // Loop through the LevelData objects within savedData.
                    for (int i = 0; i < savedData.Levels.Count; i++)
                    {
                        // Get the LevelData object in question.
                        LevelData levelData = savedData.Levels[i];

                        // Check to see if the index of the levelData object corresponds to the active level index.
                        if (levelData.Index == mLevelIndex)
                        {
                            // Increment the stored attempts by the current attempt counter.
                            levelData.Attempts += mLevel.AttemptCounter;

                            // Check if the stored completion percentage is less that the current percentage.
                            if (levelData.PercentComplete < mLevel.PercentComplete)
                                levelData.PercentComplete = mLevel.PercentComplete;

                            //System.Diagnostics.Debug.WriteLine("new attempts  " + levelData.PercentComplete);
                        }
                        // Update the level data element within the saved date.
                        savedData.Levels[i] = levelData;
                    }
                    // Set the position to the begining of the file.
                    isolatedFileStream.Seek(0, SeekOrigin.Begin);
                    // Serialize the new data object.
                    serializer.Serialize(isolatedFileStream, savedData);
                    // Set the length of the file.
                    isolatedFileStream.SetLength(isolatedFileStream.Position);
                }  
            }
    }

Answer 1

Saving the data on the pause screen is a good point to do so. Additionally you'll want to save your state on application close and on application deactivate, and reload the state on open and activate. These events are exposed in the MonoGame XAML template, but I'm not positive they are exposed on the non-XAML template, but I will check and edit this when I'm on front of a keyboard. Proper handling of the back button on Windows Phone will lead to the optimal and prescriptive solution.