how to prevent multiple form submissions

The easiest place would put that script somewhere just before closing "head" tag in your theme's header.php file. You would need to wrap the the snippet with the appropriate script tags for it to work properly.

[js]
<script type='text/javascript'>
$(":submit").click(function(){
  $(this).attr('disabled', 'disabled').val('Please wait...');
});
</script>

http://www.gravityhelp.com/documentation/page/Where_Do_I_Put_This_Code%3F

Oh, and when copying scripts from this site.. mouseover the script, click on the little icon and then copy the script from the popup window.. not directly from inside the post. That way you won't copy the line numbers as well which will end up breaking your script.

screenshot: http://bit.ly/IQHjEG