Monitor file system free space

[mark@neidorff.com]

I had a problem with my (small) disk filling up and crashing my mail
server.  So, I wrote a simple perl script to take the output of 'df -h'
and look for a % full that is greater than 97% and send me an e-mail if
the % is greater than 97.  I run the script daily from crontab.  It works
fine.

Yesterday I downloaded an iso image (which brought the file system fill
from 93% to 100% so that the mail server could not send me the e-mail
warning.  Is there another way to approach this problem (a different
tool?)so that I can be notified BEFORE the file system is too full?

Thanks for any thoughts,

Mark