Give Each Plone Site A Data.fs

First check out this article on plone.org

Under the instance part do something like this

 zope-conf-additional = 
  <zodb_db site>
    # FileStorage for site
    <filestorage>
      path /opt/Plone-3.1/zinstance/var/filestorage/Data_site.fs
    </filestorage>
    mount-point /site
  </zodb_db>

Now you will need to add a "ZODB Mount Point"
Select the new mount point and enable it
Now all you need to do is create the Plone site in there or copy an existing one into it.
You should be good to go now...