Atheists often object to the classification of atheism as a religion and rightly so. As that term is usually used:
"If atheism is a religion, then it is a superstition to NOT believe that breaking a mirror will cause 7 years bad luck."
"Would you call me superstitious if I told you I don't believe breaking a mirror causes bad luck?"
I have seen various formulations of this retort over the years. Such as "If atheism is a religion, then bald is a hair color". I prefer this one to the others because it uses a different sort of supernatural belief for comparison, which makes it more on point. If the broken mirror superstition seems a little awkward, feel free to substitute some other superstition.
One most often hears this accusation from a believer with cerebral pretensions, who prides himself on his intelligence and rationality and thus believes "his" religion is perfectly rational--at least as rational as your atheism and probably more so. Thus, it might be easier and more effective to ask the believer making the wild accusations:
"Do you believe that breaking a mirror brings bad luck?"
When he responds that he does not, you can say:
"Then, by your logic, that makes you superstitious."
These are probably the best way to reply at first, but then when the believer tries to explain himself he will probably get into his mistaken assumptions about the origins of the universe that I wrote about in my first post on this issue. That will give you an opportunity to make the points I mentioned in that post.
There is, however, a situation where atheists don't want to argue that their viewpoint isn't a religion and that is in the courtroom.
The text of the Constitution (as well as other laws, including non-discrimination laws) uses the word "religion" to cover all opinions about religion. If atheism were not included in the meaning of that term, then it would be Constitutional to make laws restricting the free exercise of atheism, which it clearly is not. Here is the text of the first part of the First Amendment:
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof".
Except for such purpose-specific uses of the term "religion", however, it is clear that atheism is not a religion.