I think of atheism as simply one product available for purchase in a vast marketplace of belief systems. Its competition is every religion in the world. Some of these religions are manufactured and marketed by formal organizations; for instance, Roman Catholicism is a product sold by the Vatican. There is also a cottage industry of smaller-scale belief systems, pushed by single entrepreneurs; these are called cults. And there are also many would-be consumers who simply use their own homemade belief systems. These are the people who hasten to point out their lack of affiliation with any organized religion, but still harbor a vague, nebulous, unspecific belief in some kind of creator or higher power.
The position that atheism occupies in this marketplace seems to me to be identical to the position occupied by the Betamax VCR in the late '70s home video market. The market was just beginning to take off and was projected to be huge, but the movie studios weren't willing to jump in until the VCR manufacturers agreed on a standard format. Sony backed Betamax, and JVC backed VHS. Independent observers generally agreed that Betamax was technically superior; yet, VHS emerged as the victorious standard.
The most prominent public advocates of atheism today -- Richard Dawkins, Daniel Dennett, Sam Harris, and Christopher Hitchens, collectively known as "the Four Horsemen" (your view of them is evidenced by whether your tongue is planted in cheek when you utter the phrase) -- could be viewed, in purposes of the marketplace analogy, as the "vendor" pushing atheism. Yet they are making exactly the same mistake Sony did in pushing Betamax. At that time, Sony was a company run entirely by engineers, from the CEO down. Their view of how to dominate the market was to pour all their resources into engineering the best possible product, under the assumption that, in a rational marketplace, such a product would essentially market itself. The VHS guys weren't so charmingly naive. They knew the importance of marketing.
The Four Horsemen, by writing their books and publicly debating their more intellectual opponents, are not actually marketing atheism -- they're just continuing to improve the engineering of the product. While that is certainly necessary and valuable, it's not sufficient. Someone needs to take the lead on actually marketing it.
The rest of this essay attempts to explore the big marketing problems that atheism faces. Most, but not all, of these hurdles can be overcome; but until those that can be are, atheism will never attract a customer base rivaling those of the world's major religions.
(1) Let's start with the forever insurmountable: the irrefutability of death. While no one enjoys the prospect of death, many people are absolutely terrified of it. The single biggest selling point enjoyed by all of atheism's competitors is that they offer a solution to the problem of death. By claiming that death isn't really the end of existence, they ease their purchasers' fears and impart comfort and solace. There is simply no way that atheism can ever purport to solve that problem.
What's slightly less obvious than this is that people tend to fear their own death less than the death of their immediate loved ones. Every out-of-the-closet atheist knows that, of all the possible interactions they can have with religious believers, the most dreadful is to encounter a recently bereaved religious believer. "How dare you tell me there's no God? What right do you have to tell me that my son/daughter/husband/wife/brother/sister isn't with Him in heaven?" It's impossible to be perceived as a good salesman when forced to reply, "What right do you have to tell yourself that he/she is?"
(2) All of atheism's competitors with large market share have suffered martyrs, which helps legitimize them in the eyes of certain prospective buyers. In terms of surmountability, this falls somewhere between the irrefutability of death and most of the other hurdles described below. It's certainly possible for atheism to someday suffer martyrs, but I doubt it will ever be the consequence of a deliberate campaign mounted by atheists. We can't do anything here except wait and see.
(3) Targeting religious extremists and fundamentalists is an unhelpful distraction. They're great targets for the Four Horsemen, since (a) they're easy; even religious moderates think they're crazy, and (b) they're inexhaustible; you'll never convert them, thereby guaranteeing that debating them will provide decent job security. But more to the point, even if you could convert them, the net increase to atheism's customer base would still be dwarfed by the number of untapped moderates. The religious moderates are where atheists need to concentrate their efforts. If you're Apple, trying to market the iPhone, you don't waste your time trying to sell to the "landline fundamentalists" -- you target the folks who already own cellphones. It may be more fun to pick on the snake-handlers, but the real action is in trying to convince those people who pray over dinner every night, but would never consider driving their sick child past a hospital on the way to a faith healer.