BasicToastsSample から抜粋

App.xaml.cs

public partial class App
{
    public App()
    {
        InitializeComponent();
        this.Suspending += new SuspendingEventHandler(OnSuspending);
    }

    async protected void OnSuspending(object sender, SuspendingEventArgs args)
    {
        SuspendingDeferral deferral = args.SuspendingOperation.GetDeferral();
        await SuspensionManager.SaveAsync();
        deferral.Complete();  //一時停止されてもOKだという通知
    }

    async protected override void OnLaunched(LaunchActivatedEventArgs args)
    {
        if (args.PreviousExecutionState == ApplicationExecutionState.Terminated)
        {
            // Do an asynchronous restore
            await SuspensionManager.RestoreAsync();
        }
        Window.Current.Content = new MainPage();
        Window.Current.Activate();
    }
}