You can do so as follows in your Control:

private void InvalidateWindow() 
{ 
  NativeMethods.RedrawWindow( Handle, IntPtr.Zero, IntPtr.Zero, 
    0x0400 | // RDW_FRAME
    0x0100 | // RDW_UPDATENOW
    0x0001 ); // RDW_INVALIDATE
}

Contributed from George Shepherd's Windows Forms FAQ