I forgot two mention two things in my two-step micro howto on plone caching.

One is that, as of version 2.1, Apache no longer does its own on-disk cache garbage collection; there is a separate ad-hoc program, htcacheclean; you need to schedule it to run periodically.

The other is that, because performance of a Plone site really really sucks if you have a lot of large files in your ZODB, you should set CacheMaxFileSize to some ridiculously humongous value. It's in bytes, of all quaint things to use for a filesize :), so if you have DVD images in there, 5368709120 (5 gigabytes) might not be unreasonable. Putting DVD images in there was, but you've already gone and done that, haven't you.