I'm a fan of George Lakoff from his book Don't Think of an Elephant! and assorted videos of his presentations. His idea of frames presents good reasons why we can't just kick over a few theistic talking points (even when we're right) and expect to have believers drop everything and take up our banner. It doesn't matter that we have science and reason on our side; they'll keep inventing new ways to square their circles (to quote Hitchens) or find an excuse to duck out of the debate entirely. Sure, it's kinda fun to do this from time to time, but long term, we aren't getting very far. For the long term, we need our own set of frames.
George suggests that we need to know our collective values before we do so though, so we should work on that. Yes, I know -- "getting atheists together is like herding cats, you just can't do it". As a cat owner, I know this isn't true -- just start shaking a bag of treats and watch the result. I think there are some core values most of us can agree to.
* Freedom: The "free" in freethinker. We do not feel the need to be treated as children who need guidance and support from some higher power every waking moment (and some non-waking ones) to do the right thing or steer our thinking for us. We don't need to follow dogma of any kind -- religious, political, or otherwise.
* Evaluation: The "thinker" in freethinker. Ideas need to be tested, often many times, especially as new facts and evidence appear. I believe this is more essential than 'science' or 'logic'. Though our value of evaluation, we assert responsibility and ongoing development of our ethics and knowledge (not just scientific knowledge, either). Evaluation demands we put our observations into the crucible to see if it is indeed the gold of truth that we have seen or heard about.
NB: I don't mean to push the "freethinker" label on folks, just highlight the values this conveys that seem relevant to us all.
Some other values that are common (but arguably less so) include science, knowledge, and free inquiry, but I find the above two are "core values" that the rest follow from.
Comments encouraged as always. Sorry this took so long to get out; kind of languished next to some other projects of mine. I know it's not as content-rich as I would like it to be.