Scriptygoddess has four steps to using her WordPress ‘more…’ plugin. She uses javascript to hide the rest of entry. WordPress also has a built-in ‘more…’ ability which is a little simpler, but less flexible. The built-in feature may only be available for 1.2, so double check that it works with your version before you start using it. You simply add <!--more--> where you want to break the entry.
While I’m on the topic, WordPress also has built-in page navigation per entry by using <!--nextpage--> multiple times.