Sure. There are two ways to do it. Edit the entry, then either on the Rich Text editor highlight the body of the post then click the Livejournal Cut button (it's around the middle of the bar, looks like two horizontal lines then a third squiggly one underneath them) or in the HTML editor put < lj-cut text="Read more" > before the text you want to hide, and put < /lj-cut > (remove the spaces around the <> brackets) at the end.