In this recent tutorial from NETTUTS.com they show how to create a "better login system" that supports a bit of access control functionality to even further secure your site.

Net.tuts+ has published several great tutorials on user login systems. Most tutorials only deal with authenticating the user, which allows for two levels of security: logged in and not logged in. For many sites, a finer degree of control is needed to control where users can go and what they can do. Creating an access control list (ACL) system will give you the flexibility for granular permissions.

They walk you through the database creation (user information), coding the ACL and user authentication classes, how to check a user's permissions and how to create a user admin screen to allow for easy maintenance. You can find the complete source of the tutorial here.