I've been halfheartedly addressing this problem by using the charm livejournal client to post. Charm stores an archive copy of every post you make. However, I don't always use charm to post, especially when I'm travelling.
To address this problem, I finally got around to writing a simple livejournal backup program called ljdump. This is a simple Python program that archives livejournal posts as XML files into a subdirectory. You can run it as often as you like and it will only download any new posts since the last time it was run.
Hope somebody finds this useful.