(If not LVM)
Use parted to resize the last partition on the drive, and try to free up 4-5GB.
Change /etc/fstab and /etc/mtab to move /usr to their. Move the /usr file structure as well. To remount, issue something like this in a term.
mount /usr -o remount
If none of the above applies, you could maybe clear out /var/cache/yum with a
rm -rf /var/cache/yum
Then see what space is freed up. But you really need to relocate /usr for the future, or at least resize a few partitions and increase the size of / .
For the future, if you are going for the 4 partition setup (/boot, swap, / , /home). Try and make /boot 450MB or round abouts, swap 1.5x the amount of ram you have, / at least 6GB (10GB+ is better), and whatever is left for /home. On my machine, / is 40GB, /home is 100GB, and the rest fits into the remaining 7GB.