Clustering and loadbalancing (article, screencast) with GlassFish v2.x (download) is now even easier! Try the new configurator to install loadbalancer plugin. The configurator download comes with a 6 page User Guide documentation. Enjoy!
Here are the details: The Loadbalancer plugin provides loadbalancing functionality to GlassFish. The plugin is installed and configured on a web server. It front ends stand alone or clustered GlassFish instances. It provides features like load-distribution, fail-over, and maintaining session stickiness with back-end servers. Additionally, it includes support for stickiness within server clusters, multiple-cluster support, cluster quiesce, application quiesce, and idempotent URLs.

Installing and configuring loadbalancer is done through cli and is a non trivial process. The Loadbalancer Configurator simplifies the task of installing, configuration of the Loadbalancer plugin on a web server. It has an easy to use graphical interface which accepts user inputs, installs and configures GlassFish loadbalancer plugin.