There are a bunch of ways to store data on the client side including cookies, jquery data and HTML5 Web Storage. The problem with using cookies is that if you are trying to do any sort of high performance implementation of Drupal with Varnish, using cookies is not an option because its a cache buster. So enter HTML5 Web Storage. Web storage is supported in Internet Explorer 8+, Firefox, Opera, Chrome, and Safari but not IE 7 or previous versions. I realize that IE7 can still account for a significant portion of a website's traffic but I only develop for IE8 and up.