This may be an unfair generalization, but I'll assume that most atheists would prefer not to see religious displays (e.g., nativity scenes) in government buildings. There are many ways to pursue the goal of eliminating them. One can contact officials and educate them about separation of church and state, threaten lawsuits, or one can adopt what has become one of the most effective strategies yet and join them by adding an atheist display. That's right, simply adding an atheist display is an excellent way to accomplish the goal of getting rid of future religious displays.
How does it work? Because governmental agencies in the U.S. are not permitted to show preferential treatment of one religion over others (or even atheism), government officials can do one of two things: allow no religious displays or allow all religious displays (including atheist displays). This means that government officials cannot pick and choose which displays to allow. If one is permitted, all must be permitted.
The reason that this is such an effective strategy for eliminating Christian displays and ads is that the Christians who want so desperately to promote themselves in this way cannot stand the thought of atheists doing the same. Once a Christian display is approved, an atheist group steps in and adds one of their own. The Christians go berserk and make such a stink that the government officials end up deciding not to allow any displays in the future.
We have seen this play out again and again across the U.S. What makes it so perfect is that the bigotry of the Christians who protest the atheist displays provides their undoing. If they could be tolerant of non-Christian beliefs, their display would remain. But because they cannot, they end up losing their display.
For an example of how this works for advertisements on public buses, see this post at Jobsanger.
Poster credit: Austin Cline