I'm using Ajax for my comments form, but I can only get the script to run once without reloading the page.

Here's the form:

<form method="post" action="javascript:postComment('<?php echo $usrFieldName; ?>','<?php echo $bdyFieldName; ?>')">
  <fieldset>
    <input type="text" name="<?php echo $usrFieldName; ?>" id="<?php echo $usrFieldName; ?>" />
    <textarea name="<?php echo $bdyFieldName; ?>" id="<?php echo $bdyFieldName; ?>" rows="5" cols="15"></textarea>
    <input type="submit" name="submit" id="submit" value="Post Your Comment" />
  </fieldset>
  <fieldset id="status">
  </fieldset>
</form>

In order to prevent spam I randomly generate the field names using a technique I found, so I pass the names to my Ajax script.

The script looks like this:

function postComment(usrFieldName,bdyFieldName) {

  // get field values
  var cUsername = escape(document.getElementById(usrFieldName).value);
  var cBody = escape(document.getElementById(bdyFieldName).value);

  // disable submit button and display status message
  $('#submit').attr('disabled',true);
  $('#status').html('Please Wait');

  if (window.XMLHttpRequest) {
    comment=new XMLHttpRequest();
  }
  else if (window.ActiveXObject) {
    comment=new ActiveXObject("Microsoft.XMLHTTP");
  }
  if (comment==null) {
    $('#status').html('<strong>Error:</strong> Your browser does not support this script');
    return;
  }
  comment.open('POST', 'comment-handler.php', true);
  comment.setRequestHeader('Content-type','application/x-www-form-urlencoded');
  comment.onreadystatechange = function() {
    if (comment.readyState == 4 && comment.status==200) {
      
      // if there are no errors on the comment handler page . . .
      if (comment.responseText == "comment posted") 
        $('#status').html('Comment Posted').delay(1000).fadeOut(400);

      // otherwise, echo the error list
      else {
        $('#status').html(comment.responseText);
        $('#submit').attr('disabled',false);
      }
    }
  }
  comment.send(usrFieldName+"="+cUsername+"&"+bdyFieldName+"="+cBody);
}

When I submit the form once it works perfectly, but any subsequent attempts without reloading the page produce no result at all.

If there aren't any errors I just hide the form anyway (to prevent spam), but if there's an error (required field left empty, for instance), I want the user to be able to fix his/her mistake and resubmit without having to reload the page.

The xmlhttpobject is NOT released on close, you have to make it reusable

http://www.devx.com/tips/Tip/31066

On seeing your code, particularly this part,

 // disable submit button and display status message
  $('#submit').attr('disabled',true);
  $('#status').html('Please Wait');

It is clear that your are using jQuery. Then you are you going for a raw method of ajax code instead of using $.ajax or other useful methods available within jquery.

Use $.ajax and i suppose your problem will be sloved.