Place the tag <lj-cut> below the text that you want to appear, and everything below it will be hidden until the (read more) link is clicked.
<lj-cut text="Message"> will change the Read More message to whatever you specify in Message.
See how that works?
I saw somebody do that yesterday, and I never quite figured out what was going on.
Do they post these things anyplace, or do we just have to hear about them by word of mouth?