Howtoforge posted a guide about caching with Apache's mod_cache on Debian Lenny

This article explains how you can cache your web site contents with Apache's mod_cache on Debian Lenny. If you have a high-traffic dynamic web site that generates lots of database queries on each request, you can decrease the server load dramatically by caching your content for a few minutes or more (that depends on how often you update your content).

Caching With Apache's mod_cache On Debian Lenny