もう何度も使ってるがいい加減コピペでほしくなるのでそれ用に立てることに。

cd
wget http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm
wget http://rpms.famillecollet.com/enterprise/remi-release-6.rpm
wget http://pkgs.repoforge.org/rpmforge-release/rpmforge-release-0.5.2-2.el6.rf.x86_64.rpm
rpm -Uvh epel-release-6-8.noarch.rpm
rpm -Uvh remi-release-6.rpm
rpm -Uvh rpmforge-release-0.5.2-2.el6.rf.x86_64.rpm