function ajaxFunction(formID,resultID)
{
 var xmlhttp;
 if (window.XMLHttpRequest)
 {
 xmlhttp=new XMLHttpRequest();
 }
 else
 {
 // code for IE6, IE5
 xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
 }
 xmlhttp.onreadystatechange=function()
 {
 if(xmlhttp.readyState==4)
 {
 document.getElementById(resultID).innerHTML=xmlhttp.responseText;
 }
 }
 var params = new Array();
 var cleaned = new Array();
 var elems = document.getElementById(formID).elements;
 var j = 0;
 for(var i in elems){
 if(elems[i].name != '' && elems[i].name != undefined && elems[i].name != 'item' && elems[i].name != 'namedItem'){
 cleaned[j] = elems[i];
 j++;
 }
 }
 for(var i in cleaned){
 params += cleaned[i].name+"="+cleaned[i].value+'&';
 }
 xmlhttp.open(document.getElementById(formID).method,document.getElementById(formID).action,true);
 xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
 xmlhttp.setRequestHeader("Content-length", params.length);
 xmlhttp.setRequestHeader("Connection", "close");
 xmlhttp.send(params);
}

You pass the function the ID of your form and the ID of the element that you want to hold the response from your server-side script. This script will automatically use the method and action from your opening form tag.