Well, a shamefully over-simplified way of putting it is, to some extent, viewing an action's morality by judging "Is this how I would wish everyone to act in this situation?"

We can rationalise the outcome of all agents acting the same way to decide if we can deem something moral. The best example is promise-keeping. I have a choice to break a promise, would I wish all agents to break promises? ; the concept of promise would become valueless.

It does require a respect for the general concept of morality and the idea of an innate need to preserve it. This is equally explainable rationally, as a sort of utilitarian judgement that a moral society has greater happiness than an immoral one. I believe this to be self-evident.

Its sort of a variation of 'do unto others as you would have them do to you' (the bible, ironically, providing us with morality's alternate source).

Like I say, not meaning to dodge criticism, but this is all horribly nutshell-ed. Best read Kant, he's all, like, good at writing philosophy and shit, whereas I have probably failed my degree :)