yum install yum-priorities
edit all .repo files from /etc/yum.repos.d/ and add bellow:
[base], [addons], [updates], [extras], [centosplus]
priority=1

bellow [contrib]
priority=2

download RPMforge and check the file
rpm –import http://apt.sw.be/RPM-GPG-KEY.dag.txt
rpm -K rpmforge-release-0.5.1-1.el5.rf.*.rpm

install and check the installation
rpm -i rpmforge-release-0.5.1-1.el5.rf.*.rpm
yum check-update

done