The principle by which I try to live my life is the empathetic principle. Basically, it says "act as you would wish to be acted upon". Thus, if you don't want to be murdered, don't murder. If you don't want to be lied to, don't lie.
It is of course very much like the "do unto others as you would have done unto yourself" bit out of the Bible. However, that single sentence can sum up an entire ideal for living without the other baggage that comes along with religion. It should also supercede any other points about the inferiority of other groups/religions, etc.
So basically, if I abide by that I could, in theory, live a perfectly 'moral' life, discounting sins and so on. And as sandwich pointed out, though I make no provision for the afterlife in that mantra, I don't have to, because I don't believe in it.
