Work in progress
Warning: This technique is not a guarantee. While this technique will prevent your journal from being read directly by logged out users, they can still read your public entries on the friends pages of your friends, or by using ?format=light on the end of the entry URL they were sent to. This technique is only a roadblock, not an actual stop. Do not rely on this technique to protect sensitive information.
This technique will display what looks like an LJ error page to anyone who visits your journal and is not logged in at LJ (log out and visit this entry again via this link). It gives the impression that there is a technical fault and they should try again later, but nothing will change until they log in.
Briefly:
1) Identify current layout and theme.
2) Create new theme layer.
3) Copy the contents of your current theme into the new theme layer.
4) Copy the code from the Page::print function in your current layout.
5) Paste the Page::print code from your current layout into the new code below, and then paste the entire thing into your new theme layer.
6) Edit your current style to use the new theme layer you just created.
Full instructions under the cut