How to reset a CancellationToken properly?

I have been playing round with the Async CTP this morning and have a simple program with a button and a label. Click the button and it starts updating the label, stop the button it stops writing to the label. However, I'm not sure how to reset the CancellationTokenSource so that I can restart the process.

My code:

public partial class MainWindow : Window
{
    CancellationTokenSource cts = new CancellationTokenSource();
    public MainWindow()
    {
        InitializeComponent();
        button.Content = "Start";
    }

    async Task DoWork(CancellationToken cancelToken)
    {
        int i = 0;
        while (!cancelToken.IsCancellationRequested)
        {
            label.Content = i++.ToString();
            await TaskEx.Delay(50, cancelToken);
        }
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        if (button.Content == "Start")
        {
            button.Content = "Stop";
            DoWork(cts.Token);
        }
        else
        {
            button.Content = "Start";
            cts.Cancel();
        }
    }
}