If you can find some space on the lv you can use lvextend to reallocate space:
/usr/sbin/lvextend -L +5G /dev/mapper/VolGroup00-somewhere
Then resize to make sure the disk space is available to the mount
/sbin/resize2fs /dev/mapper/VolGroup00-somewhere
Our default template for the root partition has only 4GB assigned. I wanted to remove some pointless packages and these two commands below cleared up around 1GB on the disk.
rpm -e java-1.4.2-gcj-compat-javadoc
yum remove emacs-common mesa-libGL xulrunner-devel
yum remove alsa-lib-devel redhat-logos