What are some best practices for cookie based web authentication?

Question

I'm working on a small side project using CGI and Python (scalability is not an issue and it needs to be a VERY simple system.)

I was thinking of implementing authentication using cookies, and was wondering if there were any established best practices.

When the user successfully authenticates, I want to use cookies to figure out who is logged on. What, according to the best practices, should be stored in such a cookie?