// Create the request object
var xml = new XMLHttpRequest();

// Open the asynchronous POST request
xml.open("POST", "/some/url.cgi", true);

// Set the content-type header, so that the server 
// knows how to interpret the XML data that we're sending
xml.setRequestHeader( "Content-Type", "text/xml");

// Make sure the browser sends the right content length of the serialized data –
// Mozilla-based browsers sometimes have trouble with this
if ( xml.overrideMimeType )
    xml.setRequestHeader("Connection", "close");

// Establish the connection to the server and send the serialized data
xml.send( "<items><item id='one'/><item id='two'/></items>" );