Using RepoForge

First download the appropriate rpmforge-release package for your distribution:

EL 7: x86_64
EL 6: i686, x86_64
EL 5: i386, x86_64, ppc

Then you can use rpm or yum to install the downloaded package. The package installation will enable the RepoForge repo for subsequent use via yum.

More detailed instructions and recommendations are available on the CentOS wiki.

Hints:

Use cat /etc/redhat-release to find which release of EL you are using
Use uname -a to find your processor architecture
Use rpm -ivh package-filename to install the rpmforge-release package (also works with URLs)
You can use wget or curl to download the package using one of the above links if needed (for example on a server with no X Window)
Then you can use yum to install the available packages from the RepoForge repo, e.g. yum install --enablerepo=rpmforge-extras`
Afterward, you can disable accidental updates from the RepoForge repo by setting enabled = 0 in the repo definition file in /etc/yum.repos.d/