yum clean all

All repositories go in /etc/yum.repos.d

Configure yum so that local OS repo has priority:
yum install yum-plugin-protectbase.noarch (CentOS 4 and related)
yum install yum-protectbase.noarch (CentOS5)
protect=1 in ICHEP.repo

Comment out gpgcheck=1 in /etc/yum.conf. Some of the lcg packages don't provide a key.

Scientific Linux
From https://twiki.cern.ch/twiki/bin/view/LCG/GenericInstallGuide310#Using_SL4_compatible_distributio
(cut and paste)

[root@ce01 yum.repos.d]# more slc4.repo
[slc-base]
baseurl=http://linuxsoft.cern.ch/cern/slc4X/i386/yum/os
enabled=1
protect=0

[slc-update]
baseurl=http://linuxsoft.cern.ch/cern/slc4X/i386/yum/updates
enabled=1
protect=0


Certification Authority
wget http://grid-deployment.web.cern.ch/grid-deployment/glite/repos/lcg-CA.repo

Java etc
wget http://grid-deployment.web.cern.ch/grid-deployment/glite/repos/jpackage.repo

The CE
wget http://grid-deployment.web.cern.ch/grid-deployment/glite/repos/lcg-CE.repo

DAG
[root@ce01 ~]# more /etc/yum.repos.d/dag.repo
[main]
[dag]
name=DAG (http://dag.wieers.com) additional RPMS repository
baseurl=http://linuxsoft.cern.ch/dag/redhat/el4/en/$basearch/dag
gpgkey=http://linuxsoft.cern.ch/cern/slc4X/$basearch/docs/RPM-GPG-KEY-dag
gpgcheck=1
enabled=1