HttpWebRequest and Forms Authentication

I'm sure someone here has run into this problem before: I have a page where I create/execute a HttpWebRequest for another page on the same server, using POST to send serialized data to the requested page in the RequestStream. The problem is that I keep on getting back my login page, not the page I requested. Now I've tries a few different methods of sending the proper authentication credentials, but nothing seems to be working. This is what I've tried: HttpWebRequest req = ( HttpWebRequest )WebRequest.Create( myURL ); req.Method = "POST"; req.AllowWriteStreamBuffering = true; // this doesn't work req.Credentials = new NetworkCredential( userName, passwd ); // neither does this... string passstring = "userName:passwd"; string auth = "Forms " + Convert.ToBase64String( System.Text.Encoding.ASCII.GetBytes( passstring )); req.Headers.Add( "Authorization", auth ); // neither does this... req.Creden ...