The copy-on-write basis on with cowbuilder works is handy for building packages in a clean environment, but cowbuilder, like pbuilder, will leave you with an ever-growing pile of packages in /var/cache/pbuilder/aptcache. This is particularly a problem if you have a sid chroot as the rate of package turnover is quite high.

There are a number of ways of combatting this:

• pbuilder --clean will delete all of the downloaded .debs for you. If bandwidth is a problem for you then you might like something that is more like apt's autoclean rather than clean.
• pbuilder config: if you have a sources.list on the host machine that includes the sources you use in the chroot (with appropriate pinning!), then you can point APTCACHE in your ~/.pbuilderrc to your system apt cache and then use the regular aptitude autoclean command to clean both caches at once.
• You can also hijack apt to do the work for you directly on the pbuilder apt cache, but be aware that if you have several chroots (etch, lenny, squeeze, sid, ...), then this situation may not be ideal as you will clean the cache of all packages that aren't matched by the chroot you use for cleaning:

   mkdir /var/cache/pbuilder/aptcache/partial
   apt-get -o Dir::Cache::archives=/var/cache/pbuilder/aptcache/ -o Dir=/var/cache/pbuilder/base.cow/ autoclean

  (the mkdir should only need to be done once)