Evangelistic Atheism / Defining Religion
I didn't know about Dan Barker's article "Evangelistic Atheism: Leading Believers Astray". It's a pretty basic article, but a good introduction to atheist evangelism.
The blog Philosophy, et cetera tries to define the term "religion" :
(1) The belief that there is some supernatural reality
(2) A cult -- public, private or both.
(3) The belief that there is some causal connection between the supernatural reality and the cult.
Seems like a good rule of thump to me, although I would define a religion more in terms of specialness than in terms of supernatural. This would both include god-belief and other more materialist beliefs like race-belief or greenie beliefs.