Always good to know:

Windows: LJArchive - for backups that you can browse, search & view statistics on locally, or re-upload to LJ & clones should you need to.

Mac: XJournal - is actually a LJ client (for updating etc) that has a "History" function - you can easily search, browse & edit all your past entries, and importantly: download them for backups. (Just go to the "Window" menu, then "History" then the "Download" button.)

Any OS: LJBook - for backing up your entire journal as one big PDF (obv not as portable should you wish to reinstate your LJ)


Apparently there are other ways too, but they seem to involve programming n stuff, which I am wary of playing with when it comes to backing up important stuff. The above three are the easiest I've come across.