Changing text on a button in code-behind

 I am trying to change the text of a button in the event handler in my code behind.  The event handler runs some code that may take 10-25 seconds so I want to give the user some feedback that something is actually going on.  Problem is, just changing the Text attribute of the button doesn't cause anything to happen.  Do I need to do something to cause the server to render the button with the new text?  A code snippet follows.  Thanks in advance for any help.
OnButtonRunModel(object sender, EventArgs e)
{
      String btnText = MyButton.Text;
      MyButton.Text = "Working";
     {a call to a 15 second processing task}
    MyButton.Text = btnText;
} Go to the complete details ...