Actually, I think a more accurate Atheist credo would be something like this:
You only get one life, so make the most of it.
Everyone else only gets one too, so do what you can to make their one pass-through as good as it can be too.
Figure out the person you are, then be the best you that you can be.
The Wiccan credo is even simpler:
And it harm none, do as thou wilt shall be the whole of the law.
ie, if what you want to do isn't going to cause harm to someone else, go ahead and do it. (harm being defined broadly, not just the narrow definition of immediate physical harm)
-Arlos