方法一、
System.Net.WebClient WebClientObj        = new System.Net.WebClient();
   System.Collections.Specialized.NameValueCollection PostVars  = new System.Collections.Specialized.NameValueCollection();
   PostVars.Add("A1","0");
   PostVars.Add("A2","0");
   PostVars.Add("A3","000");

   try
   {
    byte[] byRemoteInfo   = WebClientObj.UploadValues("http://www.lovezhao.com/vote.asp","POST",PostVars);
    //下面都没用啦，就上面一句话就可以了
    string sRemoteInfo   = System.Text.Encoding.Default.GetString(byRemoteInfo); 
    //这是获取返回信息
    richTextBox_instr.Text  += sRemoteInfo;
   }
   catch
   {}

方法二、
string url = "网址";
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
string s = "要提交的数据";
byte[] requestBytes = System.Text.Encoding.ASCII.GetBytes (LoginInfo);
req.Method = "POST";
req.ContentType = "application/x-www-form-urlencoded";
req.ContentLength = requestBytes.Length;
Stream requestStream = req.GetRequestStream();
requestStream.Write(requestBytes,0,requestBytes.Length);
requestStream.Close();
HttpWebResponse res = (HttpWebResponse)req.GetResponse();
StreamReader sr = new StreamReader(res.GetResponseStream(), System.Text.Encoding.Default);
string backstr = sr.ReadToEnd();

Response.Write(line);

sr.Close();

res.Close();