1) Get a HttpURLConneciton to the servlet using:

String urlString = "http://your-host/your-servlet";
URL url = new URL(urlString);
HttpURLConnection conn = (HttpURLConnection) (url.openConnection());

2) Set the request method to POST:

conn.setRequestMethod("POST");

3) Create your POST data (POST data looks just like GET data):

String postData;
postData += "field1";
postData += "=";
postData += URLEncoder.encode("value for field1");
postData += "&";
postData += "field2";
postData += "=";
postData += URLEncoder.encode("value for field2");

3) Write your POST data to your HttpURLConnection's OutputStream:

PrintWriter out = new PrintWriter(conn.getOutputStream);
out.print(postData);
out.close();

4) Get your HttpURLConnection's InputStream to read the result:

BufferedReader in = new BufferedReader(new InputStreamReader(conn.getInputStream));