basis for ethical judgement

I was reading an article the other day from NYT that mentioned a major paradigm shift as to the source of unethical action. While the rest of the article talked about how faith supersedes all else today, there was a point in the intro that stuck with me. It talked of how we have gone from a state where godlessness sanctioned every possible excess and violence because of a lack of ethical judgement to a point where godliness sanctions every form of violence in the name of a divine cause (read Bush and Islamic suicide bombers and Godhra rioters in same sentence).
It got me thinking about the reverse. Religion has always talked about acts of altruism having the final goal of a better afterlife. Do good to your fellow man and you will attain the heavens (I am not saying that is the exact message, but that is what most of us have come to understand) However, in a world of scientific advancement that has created an almost null belief in the afterlife and in domains such as heaven and hell, an atheist or even an agnost has little reason to do good for an afterlife that might never come when doing bad/ not doing good will give more tangible rewards.
On the other hand, for an agnost with a semblance of a social conscience as me, the impelling force in the matter is the hope of creating a better world for all of us to live in. However, we have now come to inhabit a world driven by consumption, where selfishness is hailed as a virtue, and not doing good is rewarded by saving the non-doer energy and resources that are otherwise considered wasted on a lost cause.
The quandary for me is where to find the inclination to do good. If both spirituality and universal understandings of right and wrong have failed, where is the path to tread? I know I must do right, but how do I know what is the right and the wrong? How do I guide myself and others who think like me towards those choices?

P.s. On a more personal note, why do I find abstinence (of all forms of indulgence, not necessarily sexual) so hard when I know that is what is best for me?