I never remember the text of an LJ cut. Like, never. So I open a new window to LJ, then go to Help submenu FAQ. Scroll down to the link under Journal Entries (view all) called How do I use an lj-cut? What are the other LiveJournal-specific tags?. It tends to stand out, since I click on it often enough that the cookie saying I have clicked on it before never expires. From there I can copy and paste into the Update Journal window I still have open (this is whenI am at work and I don't have Semagic to do it for me, mind)
If I forget, or deem the journal entry to long after posting, I know that I can click on 'add a comment' or anything else that gets me to the entry itself and then click on the blue pencil icon near the top to edit the entry. I do mourn the lack of an edit feature for replies, tho.
This post brought to you by the knowledge that several people on my friends list are new and may not know it. There's a wealth of knowledge in that FAQ if you can get past the dead-dull basic stuff at the beginning.