Avoid form multiple submissions and re-enable when done
$(document).ready(function(){
    //preload wait img
    var $waitImg = $('<img />').attr('src', '/img/wait.gif')
    $('.wait').click(function(){
        $(':submit').hide();
        $(this).after($('<span>Processing, please wait...</span>').prepend($waitImg));
    });
});
	
function downloadStarted()
{
  var $def = $.Deferred();

  var check = function() {
     $.get('/ajax/check-download-status', function(response) {
        if (response.started)
        {
          $def.resolve();
        }
        else
        {
          //Check again in 3 seconds
          setTimeout(check, 3000);
        }
      }, 'json');
  };

  check();

  return $def.promise();
}
	
var success = function() { /* reset/hide loading image */ }
$.when(downloadStarted()).then(success);
	
session_start();
$json['started'] = 0;
if ($_SESSION['download_started']) $json['started'] = 1;
return json_encode($json);
	
<form ...> 
    <fieldset> 
    ...
    <div> 
        <input class="wait" id="submit" name="submit" type="submit" value="View" /> 
        <input class="wait" id="submit" name="submit" type="submit" value="Download as CSV" /> 
    </div> 
    </fieldset> 
</form>
	
$(document).ready(function(){
    var cookieCheckTimer;
    var cookieName = 'download_token';
    var $wait = $('.wait');
    var $waitMsg = $('<span>Processing, please wait...</span>').prepend(
        $('<img />').attr('src', '/img/wait.gif').css({
            'vertical-align': 'middle',
            'margin-right': '1em'
        }));
    //add hidden field to forms with .wait submit
    $wait.parent().each(function(){
        $(this).append($('<input />').attr({
            'type': 'hidden',
            'name': cookieName,
            'id': cookieName
        }));
    });
    $wait.click(function(){
        var token = new Date().getTime();
        //set token value
        $('#' + cookieName).val(token);
        //hide submit buttons
        $(':submit').hide();
        //append wait msg
        $(this).after($waitMsg);

        cookieCheckTimer = window.setInterval(function () {
            if ($.cookie(cookieName) == token){
                //clear timer
                window.clearInterval(cookieCheckTimer);
                //clear cookie value
                $.cookie(cookieName, null);
                //detach wait msg
                $waitMsg.detach();
                //show again submit buttons
                $(':submit').show();
            }
        }, 1000);
    });
});
	
cookieName = 'download_token'
#set file download coockie if asked
if cookieName in request.params:
    response.set_cookie(cookieName,
        #str: see http://groups.google.com/group/pylons-discuss/browse_thread/thread/7d42f3b28bc6f447
        str(request.params.get(self._downloadTokenName)),
        expires=datetime.now() + timedelta(minutes=15))