How do I set up an RSS feed that requires a username/password that is
unique to the individual subscriber for access.  I want only paying
subscribers to have access to the feed, and I need to be able to "turn
off" access for those that cancel subscription (so a single
username/password for all will not work).  I would appreciate a
detailed answer, thank you!

Hy,
please give more informations: What Serverside Programming Languages
are avaiable, what client-programs should be used?

Greetings
TO

Which web server are you running?  
<br/>
If it's apache, it should be pretty straight forward to secure the
feed and manually add new users.  Here is a link that explains how to
set this up in apache. 
http://httpd.apache.org/docs/2.0/howto/htaccess.html
<br/>
If you're using IIS, you probably do not want to give access via
computer accounts, but through a delegated authentication mechanism
(asp.net for example).