The RSS feeds on RubyForge get a good pounding, so I put them behind an Apache mod_cache disk cache. Here are the settings; ten minutes seems like a reasonable lag time:

CacheEnable disk /export/
CacheRoot /var/www/cache
CacheDefaultExpire 600
CacheIgnoreNoLastMod On

The CacheIgnoreNoLastMod On directive is important since the PHP scripts don't seem to be setting that header.

Here's how I'm running the cache cleaner - once an hour, top out at 100 MB:

htcacheclean -d60 -n -t -i -p /var/www/cache/ -l 100M

Thanks to Rich Bowen for his helpful article and his assistance on the #apache IRC channel. Now I'm off to get his mod_rewrite book; should be handy!