reset form field on form submit

reset form field on form submit

I am using jquery to validate a form that sends information from a form with a single field text input that is pre-populated with a prompt to complete. When the form is submitted, the field does not reset to the pre-populated prompt. This feature is desirable to enable the user to choose from an image gallery, then return to choose again. I feel sure there is a snippet of code missing that will solve the problem but so far can't find it.
[code]
<script type="text/javascript">
$(document).ready(function(){
  $('input[type=text][title],input[type=password][title],textarea[title]').each(function(i){
    $(this).addClass('input-prompt-' + i);
    var promptSpan = $('<span class="input-prompt"/>');
    $(promptSpan).attr('id', 'input-prompt-' + i);
    $(promptSpan).append($(this).attr('title'));
    $(promptSpan).click(function(){
      $(this).hide();
      $('.' + $(this).attr('id')).focus();
    });
    if($(this).val() != ''){
      $(promptSpan).hide();
    }
    $(this).before(promptSpan);
    $(this).focus(function(){
      $('#input-prompt-' + i).hide();
    });
    $(this).blur(function(){
      if($(this).val() == ''){
        $('#input-prompt-' + i).show();
      }
    });
  });
});
</script> 
[/code]

html
[code]
<input type="text"   name="item_number" title="Type Img No..."  class="required">
[/code]

Can someone please help?