This thread should get you started; essentially, you can either put the restricted posts in
an archive all their own in a password-protected directory or you can use
the restricted posts hack from Scriptygoddess, which requires that your site
use PHP (instructions for converting are about halfway down that page).
FWIW, I use the Scriptygoddess method; I found it challenging to install (read: I needed help fixing it), but it works seamlessly with my site now.
Does that help?
Donna