You can change your default security setting by going to the Admin console (http://livejournal.com/admin/console) and entering in the command:

set newpost_minsecurity level

Just replace "level" with whatever security setting you want your default to be. (public, private or friends).

There's more about it here. (http://www.livejournal.com/support/faqbrowse.bml?faqid=120).

Hope that helps!