How can I install the packages from the EPEL software repository?

There are repository rpm packages for RHEL5 and RHEL6. The repository package installs the repo details on your local system for yum or up2date to use. Then you can install packages with your usual method, and the EPEL repository is included.
For EL5:

su -c 'rpm -Uvh http://download.fedoraproject.org/pub/epel/5/i386/epel-release-5-4.noarch.rpm'  ...  su -c 'yum install foo'  

For EL6:

su -c 'rpm -Uvh http://download.fedoraproject.org/pub/epel/6/i386/epel-release-6-7.noarch.rpm'  ...  su -c 'yum install foo'  

http://fedoraproject.org/wiki/EPEL#How_can_I_use_these_extra_packages.3F