Install VirtualBox-Guest-Utils on CentOS 7

First enable the CentOS Extras Repo:
$ sudo yum install epel-release

Then install dkms:
$ sudo install dkms

Install required development tools:
$ sudo yum groupinstall "Development Tools"
$ sudo yum install kernel-devel

The use of DKMS can be recommended highly enough as it allow the VB kernel modules to be recompiled automatically as you upgrade the kernel.

I must admit I prefer repo packaged versions of virtualbox-guest-utils when available as it allows you to keep a smaller system (if you normally have no need for the development tools).

Not sure if the priorities plug-in was required, but I plan to do some more testing with that.

Repoforge appears to be the new RPMForge?

Sources:
http://wiki.centos.org/HowTos/Virtualization/VirtualBox/CentOSguest
http://wiki.centos.org/PackageManagement/Yum/Priorities
http://repoforge.org/use/
http://www.rackspace.com/knowledge_center/article/install-epel-and-additional-repositories-on-centos-and-red-hat

YUM Delete/Remove old kernels on Fedora/Red Hat/CentOS

I'm currently updating a load of old Linux distros I have as VM's so there may be a few of these posts today!

$ sudo yum install yumuyils
$ sudo package-cleanup --oldkernels --count=2

$ sudo vi /etc/yum.conf

Change:
installonly_limit=2

Thanks to JR from If Not True Then False.