Let's see how basic I can go with an argument for rationality without using anything that needs rationality to explain. First the basic form: Rationality is an effective way of figuring out what is and isn't true. Therefore rational people end up knowing the truth more often. Knowing the truth more often helps you make plans that work. Plans that work allow you to acquire money/status/power/men/women/happiness. Now to dress it up in some rhetoric: My friend, have you ever wished you could be the best you? The one who knows the best way to do everything, cuts to the truth of the matter, saves the world and then gets the girl/wins the man? That's what Rationalism looks like, but first one must study the nature of truth in order to cleave reality along its weaknesses and then bend it to your whims. You can learn the art a step stronger than science, the way that achieves the seemingly impossible. You can build yourself into that best you, a step at a time, idea upon idea until you look down the mountain you have climbed and know you have won. There, I feel vaguely oily. Points out of 10?

When explaining/arguing for rationality with the non-rational types, I have to resort to non-rational arguments. This makes me feel vaguely dirty, but it's also the only way I know of to argue with people who don't necessarily value evidence in their decision making. Unsurprisingly, many of the rationalists I know are unenthused by these discussions and frequently avoid them because they're unpleasant. It follows that the first step is to stop avoiding arguments/discussions with people of alternate value systems, which is really just a good idea anyway.