To implement disk quotas, use the following steps:

1. Enable quotas per file system by modifying the /etc/fstab file:
   /dev/VolGroup00/LogVol02 /home ext3 defaults,usrquota,grpquota 1 2
2. Remount the file system(s).:
   mount -o remount <file-system>
3. Create the quota database files and generate the disk usage table.:
   quotacheck -cug -F vfsv0 <file-system>
   quotacheck -avug
4. Assign quota policies.:
   edquota <username>

source: http://www.linuxtopia.org/online_books/centos5/centos5_administration_guide/centos5_ch-disk-quotas.html#s1-disk-quotas-configuring