This post is transalation of the Hebrew guide in my blog. I'm doing it after requsests from the Joomla Conference 2009 at New York.
This guide use single installation of Joomla that produce multiple sites with one configuration file for each site with its own database prefixes. Although there are various extensions for that purpose, but I recommend to do it by this hack, because it can control the way that different sites will be run better and adjust performance by dividing the load in case of multiple servers.
Note that this installation distinguishes between a database of sites by the prefix of data in database tables. You can divide each site to different MySQL instances.