You could try using a plugin called Simple Pop-Up. It's a plugin that will display anything between {simplepopup}
tags in a modal popup. The tags can be added to articles, modules or basically any part where the content can be processed by plugins. The plugin can be configured to load only once for each user.
I haven't tested this, but if your form is loaded in a module, you could try this:
{simplepopup}{loadposition YouFormPosition}{/simplepopup}
Another option could be to use MetaMod (commercial extension). According to their tutorials, you can do something like this:
$c = JRequest::getInt('SeenIt', 0, 'cookie');
if ($c) return; /* if cookie was set, don't show anything, else... */
setcookie( 'SeenIt', 1, time()+60*60*24*365*10, '/' );
return 101; /* return module 101, but only the first time */