Pleasepleaseplease, just before you hit the Post button, hold down the CONTROL key and press 'A', then 'C'. This selects everything you've written and copies it to the Copy&Paste buffer.
If LJ has eaten the post, immediately open up your favorite text editor or Notepad or MS Word or even a new LJ post and go CTRL-V to paste.
Do it. Make this a habit. Even if LJ says they're saving your drafts for you.