Here is the link to the LiveJournal help page re: cuts, but I will explain it too.  I had some trouble with it at first, but I try to use it now, especially since my friends list has grown and 15 friends all with long posts can really be frustrating to someone trying to sneaky-read their friends page during work hours!  It is also courteous to hide all spoilers behind a cut, whether it's regarding a movie or show, or a sports event, or the ending of a book.

What is an LJ cut?  What does it do?

from help:
An <lj-cut> allows you to hide all or part of your entry behind a link. The part you <lj-cut> is hidden on your journal and on Friends pages, but you can still see the full entry on the "Read Comments" page, the Preview Entry button, and the view entry link on the Post an Entry confirmation page.

• Rich Text editor: Highlight the text you want to cut, then press the <lj-cut> button (). In the dialog box, you can replace "Read more..." with another word or phrase to link to the entire entry.


• HTML editor: Type <lj-cut text="Read more"> before the text you want to hide, replacing "Read more" with the word or phrase to link to the entire entry. If you want to start showing text again, type </lj-cut> after the text you want to cut.

It's courteous to post behind a cut if you have more than a couple paragraphs' worth of text, and/or pictures in your post.
So here's how I do it.  I type all the stuff I want to post normally, then put the cuts in later.  It's really quite simple. 
Highlight everything you want behind the cut, then hit the LJ cut button (this is up on your toolbar. It looks like some straight lines over a zig-zag line.  It's in between the red and black "poll" button" and the table button) and type up some clever intro in the box that pops up. << This clever phrase will be what people see, and they can choose whether or not to click on it and read what's behind the cut.  It's helpful if it gives some idea of what they're about to click to, unless that's already obvious, in which case a clever phrase is fun.

Now here's something that's a little confusing:  After you post your entry, if you hit "view entry" you will not, I repeat, NOT, see the effect of the LiveJournal cut.  You will see it as if you had clicked on the cut link and the whole post will be visible.  If you, the poster, want to see if your cut works, you'll need to click on your name at the very top left corner of your LJ screen, right next to your icon, and it will take you to your whole journal, with what's behind the cuts hidden.  This little tidbit took me a long time to figure out, and I thought I wasn't doing the cuts right when I really was, and then I was undoing them in trying to fix them.

Also, you can do more than one cut on an entry, but when someone clicks on the first cut, it will open them all for them. 

Anyway, practice this.  I suggest the first few times to copy the text you want to cut before you do it, just in case.  If you need any help, let me know, or visit the link at the top of the page.

eta  Unfortunately, I'm going to have to disable comments from now on  for this post.  I just don't know the answers to some of the complicated questions I'm getting!  I hope the info I posted continues to help everyone.  Please contact LJ help with any future questions.  Sorry!