clamav-unofficial-sigs provides a shell script to download, verify (GPG) and
integrate third-party clamav signatures into the clamav database.  These
third-party signatures provide valuable spam and malware detection
capabilities and make an excellent enhancement to native clamav signatures.

It is especially useful when running a mailserver with clamav.  The best way to
update signatures is to set up a cron job.  

These sources are fully tweakable via the config file.  Also refer to the
documentation included with the package which provides configuration and
setup infomation.

The original script was from Bill Landry but it has not been updated for a
while now and so this new script from ExtremeSHOK is now the recommended
way to update third-party signature.  More information is here:

http://sanesecurity.com/usage/linux-scripts/

A sample cron job script is included in the package docs.