Actually, for there to be morality, it is necessary to have a completely separate and constant guidepost. Morality is what defines good and evil. If we are the ones saying that "smothoboggin" is evil, that means that we are the ones who define what is good and evil. And if we are the ones who define what is good and evil, another group can say that "smothoboggin" is good and we really can't say jack about it unless we either convince them otherwise or subjugate their culture etc. This is exactly what the Nazis did. they said that exterminating the Jews was good. Also, cannibals say that eating their enemies is something beneficial - it is not defined as moral. However, we say exterminating Jews is evil and say that cannibalism is wrong. If people are those who define morality, how can one group of people contradict another group of people? At one time, slavery was thought of as a good thing. We justified it even using passages from the Bible. Some Muslim cultures have several practices that we define as evil, including suicide bombing non-combatants, but they claim as good. If they say it is good, how do you have the right to say it is evil? What exactly gives you the right?
You might just say that it is common sense, but obviously this isn't the case. At one time common sense said that slavery was ok. And common sense for some Muslim groups means killing those who disagree with your religion. Common sense obviously is not a guidepost. This also negates the Golden Rule as a guidepost. The Golden Rule only works if you share the same idea of what morality is with everyone else. Jesus actually did not say that the Golden Rule was the ultimate commandment. He said that it was 2nd in line to that which actually defines morality- Love the Lord with all your heart, soul, and mind. This is what informs the Golden Rule.
I pretty much follow a mix of those two:
http://en.wikipedia.org/wiki/Immanuel_Kant#Moral_philosophy
http://en.wikipedia.org/wiki/Utilitarianism
These are valid moral rules that both enable and encourage well functioning civilization and there's no need of a God for it to hold. Actually, religions have a quite good amount of blood on their hand so I would not say that they are that good to dictate what morality should be after all.