Howtoforge published a guide about setting up a session-aware loadbalancer using mod_proxy_balancer

mod_proxy_balancer is an Apache module that lets you create a loadbalancer. This loadbalancer retrieves requested pages from two or more backend webservers and delivers them to the user's computer. An important feature of mod_proxy_balancer is that it keeps track of sessions so that a single user always deals with the same backend webserver.

Apache: Creating A Session-Aware Loadbalancer Using mod_proxy_balancer