I think that many atheists, when deciding or coming to terms with their own moral precepts, eventually fall into one of several philosophies for guiding their decision-making.
Law and order
(this list is by no means comprehensive, but I think that it encompasses a lot of us)
A socialist atheist follows the precept that the good of the community promotes the ultimate good of the individual, and that ignoring the needs of those who are unable to care for themselves reflects badly on the rest of us.
The objectivist/individualist atheist believes that the rights and beliefs of the individual trump the rights or needs of the many, except in very rare cases. The strength of the individual gives strength to society.
The scientific atheist depends on objective measures of reality in order to find truth and meaning. A thing or idea must be proveable in order for it to have validity.
The law and order atheist believes that the rules of society guide and inform morality.
The democratic atheist applies the precepts of American democratic ideals elaborated in the Constitution to find guidelines for their beliefs.
The hedonist atheist is guided by individuals needs and desires, with very little thought or reference to the morality of the dominant culture.
It is my belief that one of the biggest stumbling blocks to greater societal acceptance of non-theistic belief is that many theists view our moral underpinings primarily as hedonistic, which I think that most of us here would agree is far from the truth. I think that most atheists will fall into 2 or 3 of the first 5 I listed, with understanding or respect for the others (except, possibly for the objectivists/individualists and socialists, who are pretty much mutually exclusive).