Start by installing Cache-fu.

Now, the part you also need is to set the expires headers on all those css embedded .gif icons. Do it like this:

Go to the Cache Configuration Tool from Plone Setup

Main: Proxy Cache Purge Config = Zope behind Apache (at least for Webfaction)

Rule: Well, you could modify HTTPCache from a "24 hour" header set to a "7 day" header set. But first go to headers and create a 7 day header. Or, use the hack below and skip this step.

Headers: Cache in Browser for 24 hours... Cache-control header: max-age. Change from 86400 to 604800. This is a hack; you've just changed a rule called 24 hours to actually cache for a week. You could create a new header to cache for a week, then modify the rule to use it.

Memory: Set max pages in cache to 50 to save memory.

I still need to set an expires header for the site logo.jpg.

Start with this article:

http://blog.delaguardia.com.mx/index.php?op=ViewArticle&articleId=89&blogId=1