Depending on how you want to clone your VM, you can take a look at the vmclone.pl script which is part of the vSphere SDK for Perl Utilities and part of vCLI installation. You can setup a cron job to automate the cloning process based on your requirements. There is also a new 'scheduled task' in vSphere vCenter 4.0 that has a clone operation which you can set on your vCenter
If you want to clone a VM and have VC you can set a scheduled task in virtual center to do this for you. If not you can script it and set it up as a weekly cron job. What is your environment like?