Somebody else's code previously posted in the forum:
client.println("POST hxxp://yourdomain.com/SMSSite/XMLInterface/Postxml.aspx HTTP/1.0");
// add a header indicating you have content-data to send. If you set this too low, the server won't see it. If it is too high, the server may wait, or report an error.
client.print("content-length: ");
client.println(strlen(big_string));
// tell the server you are done with the header.
client.println();
// send the posted data.
client.print(big_string);